| Figure 4.1: Partial qualifier hierarchy for @FlowSources. |
SPARTA! |
SPARTA is a research project at the University of Washington funded by the DARPA Automated Program Analysis for Cybersecurity (APAC) program.
SPARTA aims to detect certain types of malware in Android applications, or to verify that the app contains no such malware. SPARTA’s verification approach is type-checking: the developer states a security property, annotates the source code with type qualifiers that express that security property, then runs a pluggable type-checker [PAC+08, DDE+11] to verify that the type qualifiers are right (and thus that the program satisfies the security property).
The Checker Framework is a pluggable type-checker that provides the foundation for the SPARTA project. For more information on pluggable type-systems, please consult the Checker Framework manual at http://types.cs.washington.edu/checker-framework/.
You can find the latest version of this manual in the sparta-code version control repository, in directory sparta-code/docs. Alternately, you can find it in a SPARTA release at http://types.cs.washington.edu/sparta/release/, though that may not be as up-to-date.
If you have trouble, please email either sparta@cs.washington.edu (developers mailing list) or sparta-users@cs.washington.edu (users mailing list) and we will try to help.
This chapter briefly describes how to install the SPARTA project tools.
If using Eclipse, go to Help → Install New Software and install the Android ADT Plugin (https://dl-ssl.google.com/android/eclipse) and MercurialEclipse (http://cbes.javaforge.com/update).
gson.jar=$basedir/lib/google-gson-2.2.2/gson-2.2.2.jar
The SPARTA project needs to be updated to your development environment, so that the path to the Android SDK is correct and the correct version of Android is used. (More details about updating a project are available here: http://developer.android.com/tools/projects/projects-cmdline.html#UpdatingAProject.)To update the project with your Android settings run the following:
ant -buildfile $SPARTA_CODE/build.local.xml
Alternatively, you can run:
$ANDROID_HOME/tools/android update project --path . --target android-15
export CHECKERS=$HOME/checker-framework/checkers/ ant jar
As a sanity check of the installation, run
ant all-tests
You should see “BUILD SUCCESSFUL” at the end.
This section explains how to setup an Android application for analysis with the SPARTA tools.
1.) Ensure the following environment variables are set.
2.) If your Android project does not have a build.xml file, update the project.
$ANDROID_HOME/tools/android update project --path .
3.) Add the SPARTA build targets to the end of the build.xml
file, just above </project>.
<property environment="env"/>
<dirname property="checkers_dir" file="${env.CHECKERS}"/>
<basename property="checkers_base" file="${env.CHECKERS}"/>
<dirname property="sparta-code_dir" file="${env.SPARTA_CODE}"/>
<basename property="sparta-code_base" file="${env.SPARTA_CODE}"/>
<import file="${sparta-code_dir}/${sparta-code_base}/build.include.xml" optional="true"/>
SPARTA contains four independent tools. The first three are used to find potentially malicious code locations in an Android app. The final tool is used to statically verify information flow properties. All four tools can be invoked via ant targets; Section 2.4 explains how to install these targets in an Android app.
This tool reports uses of potentially dangerous APIs. These include uses of reflection, randomness, thread spawning, the ACTION VIEW intent, hard-coded strings such as URIs, and so forth. These APIs may be innocuous, but a human should examine their use. The file src/sparta/checkers/suspicious.astub contains the classes and methods that are considered suspicious.
ant reportsuspicious
This tool indicates every API call in the Android app that might require a permission. The file src/sparta/checkers/permission.astub contains the Android API that has been annotated with @RequiredPermissions and is used by this tool.
ant reqperms
This tool reports uses of Android and other APIs. These APIs are not suspicious in general. However, they do help the analyst to better understand the structure of the code not just with respect to its standard module structure, but in terms of how it interacts with Android interfaces.
ant reportapiusage
This tool ensures that there is no data flow in the application beyond what is expressed in the given flow policy. A full description of the Flow Checker is given in Chapter 4.
ant -DflowPolicy=myflowpolicy flowtest
This chapter gives a brief overview of the Flow Checker, a type-checker that tracks information flow through your program. It gives a guarantee that there is no information flow beyond what is expressed in the flow policy and type annotations.
The Flow Checker does pluggable type-checking of an information flow type system. It is implemented using the Checker Framework. To better understand pluggable type-checking and the Checker Framework, please consult the Checker Framework manual at http://types.cs.washington.edu/checker-framework/.
You write the annotation @FlowSources on a variable’s type to indicate what sensitive sources can affect the variable’s value. You write the annotation @FlowSinks to indicate where (part of) the value might be output.
As an example, suppose there is a declaration
@FlowSources(FlowSource.LOCATION) @FlowSinks(FlowSink.NETWORK) double latitude;
The @FlowSources(FlowSource.LOCATION) annotation indicates that the value of latitude might have been derived from location information. It does not guarantee that the value came from authentic GPS data, but only sets a bound on where the information might have come from; this assignment is legal: latitude = 47.6097. Similarly, the annotation @FlowSinks(FlowSInk.NETWORK) marks that the string latitude might be output to the network. It is also possible that the data has already been output.
The argument to @FlowSources (and @FlowSinks) is an enum constant, or a set of them to indicate that a value might combine information from multiple sources (or might flow to multiple locations). FlowSources specifies data sources such as phone number, location, etc. FlowSinks specifies sinks, such as files, network, and so on. The types of FlowSources and FlowSinks are listed in the FlowSources.java and FlowSinks.java files in sparta.checkers.quals.
Location Default Flow Type Fields @FlowSources(FlowSource.LITERAL) Method parameters @FlowSources(FlowSource.LITERAL) Return values @FlowSources(FlowSource.LITERAL) null @FlowSources @FlowSinks(FlowSink.ANY) Literals @FlowSources(FlowSource.LITERAL) @FlowSinks(FlowSink.CONDITIONAL) Local variables @FlowSources(FlowSource.ANY) @FlowSinks @FlowSources(α) @FlowSources(α) @FlowSinks(ω), ω is the set of sinks allowed to flow from all sources in α @FlowSinks(ω) @FlowSources(α) @FlowSinks(ω), α is the set of sources allowed to flow to all sinks in ω
Table 4.1: Default types
To reduce the number of annotations needed, default types are used. Table 4.1 shows the default types used by the flow checker.
The defaults are not applied if the programmer uses annotations. For example, the parameter below is of type @FlowSources(FlowSource.LOCATION) @FlowSinks(FlowSink.NETWORK) rather than @FlowSources(FlowSource.LITERAL). Note that @FlowSources means @FlowSources({}) and @FlowSinks means @FlowSinks({}).
public void sendToInternet(
@FlowSources(FlowSource.LOCATION) @FlowSinks(FlowSink.NETWORK) String message){...}
The value null has the bottom type (@FlowSinks(FlowSink.ANY) @FlowSources), so that it can be assigned to any type. For local variables, the default applies to the top level of the local variable type, but not to generic type arguments and array elements, if any.
The Checker Framework supports flow-sensitive type refinement. Assignments (such as initializers) are used to refine the type to a more precise one. Thus, in general you do not have to write type annotations on local variables. For details, see section “Automatic type refinement (flow-sensitive type qualifier inference)” in the Checker Framework manual.
If the programmer specifies only flow sources, the flow sink is defaulted to be the sinks that the all the specified flow sources can flow to. This is to say that it is the intersection of the set of sinks each source can flow to. In other words, if a type is annotated with @FlowSources(α), where α is a set of sources, then the flow sinks are the set ω where ω is the intersection of sinks B where A → B and A is a flow source in α. For example, if the flow policy contains the following:
A -> X,Y B -> Y C -> Y
then these are equivalent:
@FlowSources(A) == @FlowSources(A) @FlowSinks({X, Y})
@FlowSources(B) == @FlowSources(B) @FlowSinks(Y)
@FlowSources({B,C}) == @FlowSources({B,C}) @FlowSinks(Y)
@FlowSources(A) @FlowSinks(Y) == @FlowSources(A) @FlowSinks(Y)
@FlowSources({A,B}) == @FlowSources({A,B}) @FlowSinks(Y)
Similarly, if the programmer only specifies flow sinks, the flow sources are defaulted to be the sources that are allowed to flow to all the specified sinks. In other words, if a type is annotated with @FlowSinks(ω), where ω is a set of sinks, then the flow sources are the set α where α is the intersection of sources A where A → B and B is a flow sink in ω. For example, using the same policy file as above, the following are equivalent:
@FlowSinks(X) == @FlowSources(A) @FlowSinks(X)
@FlowSinks(Y) == @FlowSources({A,B,C}) @FlowSinks(Y)
A flow policy is a list of all the flows that are permitted to occur in an application. Flow policies are specified in a flow-policy file that is a list of flow source and flow sink pairs. If a flow is not listed in the flow policy, then it is forbidden to occur. If no flow policy is specified, then no flows are permitted.
The Flow Checker guarantees that there is no information flow except for what is explicitly permitted by the policy file. If a user writes a type that is not permitted by the policy file, then the flow checker issues a warning even if all types in program otherwise typecheck.
For example, this variable declaration
@FlowSource(FlowSource.CAMERA) @FlowSink(NETWORK) Video video = ...
is illegal unless the the policy file contains:
CAMERA -> NETWORK
Here is another example. The flow policy file contains:
ACCOUNTS -> EXTERNAL_STORAGE, FILESYSTEM ACCELEROMETER -> EXTERNAL_STORAGE, FILESYSTEM, NETWORK
The following variable declarations are permitted:
@FlowSources(FlowSource.ACCOUNTS) @FlowSinks(FlowSink.EXTERNAL_STORAGE) Account acc = ... @FlowSources(FlowSource.ACCELEROMETER, FlowSource.ACCOUNTS) @FlowSinks(FlowSink.EXTERNAL_STORAGE, FlowSink.FILE_SYSTEM) int accel = ...
The following definitions would generate “forbidden flow” errors:
@FlowSources(FlowSource.ACCOUNTS) @FlowSinks(@FlowSink.NETWORK) Account acc = ...
@FlowSources({FlowSource.ACCELEROMETER, FlowSource.ACCOUNTS})
@FlowSinks({FlowSink.EXTERNAL_STORAGE, FlowSink.FILESYSTEM, FlowSink.NETWORK})
Each line of a policy file specifies a source-to-sink(s) flow(s) that is permitted. So if “A → B,C” appears in the file, then information from source A is allowed to flow to sink B or sink C.
For example, MICROPHONE → NETWORK implies that MICROPHONE data is always allowed to flow to NETWORK. The source must be a member of the enum sparta.checkers.quals.FlowSources.FlowSource and the sink must be a member of the enum sparta.checkers.quals.FlowSinks.FlowSink. The source and sink names should not be preceded by the name of the enumeration which contains them. ANY is allowed just as it is in @FlowSources and @FlowSinks, but empty, {}, is not allowed.
Multiple sources or sinks can appear on the same line if they are separated by commas. For example, these two policy files have the same meaning:
MICROPHONE -> NETWORK, LOG, DISPLAY
is equivalent to this policy file:
MICROPHONE -> NETWORK MICROPHONE -> LOG MICROPHONE -> DISPLAY, NETWORK
The policy file may contain blank lines and comments that begin with a number sign (#) character.
To use a flow-policy file when invoking the Flow Checker from the command line pass it the option:
-AflowPolicy=mypolicyfile
Or if you are using one of the ant targets, you can pass the option to ant:
ant -DflowPolicy=mypolicyfile flowtest
Remember, not specifying a flow policy file is equivalent to not allowing any flows.
Figure 4.1: Partial qualifier hierarchy for @FlowSources.
Figure 4.2: Partial qualifier hierarchy for @FlowSinks.
As with standard Java types, the type annotation hierarchy indicates which assignments, method calls, and overriding are legal. Figure 4.1 shows part of the @FlowSources qualifier hierarchy. The top type is @FlowSources(ANY), which is shorthand for listing every source. It would be legal to annotate every variable in a program with @FlowSources(ANY), because every variable is derived from some subset of all flow sources. But, such an annotation would be imprecise. @FlowSources({}) or @FlowSources() is the bottom type, and may only be applied to variables whose value does not depend on any sensitive source.
Figure 4.2 shows part of the @FlowSinks qualifier hierarchy. The top type is @FlowSinks({}) or @FlowSinks(), which indicates that the value is only used locally by the application and never flows to any sensitive sink. The bottom type, @FlowSinks(FlowSinks.ANY), is a value that might be output to any location whatsoever; it can be thought of as a completely public value.
Note the different subtyping behavior for sources and sinks.
The Flow Checker considers conditional expressions to be a flow sink. If a variable will be used in a conditional, then it must have a flow sink of CONDITIONAL. By default, any source is allowed flow through a conditional. That is to say that ANY → CONDITIONAL is added to the flow policy by default.
Conditionals are treated this way because they can leak information. For example, the given a flow policy of USER_INPUT → FILESYSTEM, the following type checks.
@FlowSources(FlowSource.USER_INPUT) @FlowSinks(FlowSink.FILESYSTEM)
int creditCard = getCCNumber();
final int MAX_CC_NUM = 9999999999999999;
for (int i = 0 ; i < MAX_CC_NUM ; i++){
if (i == creditCard)
sendToInternet(i);
}
To catch this sort of information leak, pass -Alint=strict-conditional to change the default from ANY → CONDITIONAL to LITERAL → CONDITIONAL.
Programmers should not use @FlowSources({}) and @FlowSinks({}) for any types. These types are only needed for top/bottom types which are used in the default types: the null literal uses the bottom type in order for it to be assignable everywhere; local variables use the top type which will be refined by flow sensitivity. Every value should either flow from a literal or from some sensitive source. Likewise, every value must flow to a sensitive sink or to a conditional expression. Any variable that does not have a flow source or a flow sink does not actually affect the output of the program and should therefore be removed.
This may seem overly strict, but a variable without a flow source or flow sink that does affect the output of the program comes from an abuse of the type system. Most likely a variable with no source or sink would come from an improperly suppressed warning. Therefore it is necessary to not allow flows from and to nowhere. (The flow policy ensures this; see section Section 4.3)
Note that this does not mean you must specify both a flow source annotation and a flow sink annotation as explained in Section 4.2.
Sometimes it might be necessary to suppress warnings or errors produced by the Flow Checker. This can be done by using the @SuppressWarnings("flow") annotation on a declaration. Because this annotation can be used to subvert the Flow Checker, its use is considered suspicious. Anytime a warning or error is suppressed, you must write a brief comment justifying the suppression. @SuppressWarnings("flow") should only be used if there is no way to annotate the code so that an error or warning does not occur. Most programs should not suppress warnings.
Files in sparta-code/src/sparta/checkers/flowstubfiles provides library annotations. You may need to enhance them, if you find that your application uses APIs that are not yet annotated. For details, see section 6.2.8 of this manual, and also chapter “Annotating libraries” in the Checker Framework manual.
Two additional type annotations can be used to annotate polymorphic methods: @PolyFlowSources, @PolyFlowSinks.
To make the type system more expressive, the flow type system supports qualifier polymorphism, via the type qualifiers @PolyFlowSources and @PolyFlowSinks. These are mostly used when annotating APIs when the specific flow sources or flow sinks are not known or can vary. See section “Qualifier polymorphism” in the Checker Framework manual.
Three additional declaration annotations can be used to annotate APIs: @DefaultFlow, @ConservativeFlow, and @PolyFlow. They change the default annotations (Section 4.2).
The declaration annotations can be used on any declaration: a method, a class, or even a whole package. For example, the whole android package should use conservative defaults. More specific annotations given in the rest of the file override these defaults.
An element annotated with @DefaultFlow expresses that an enclosed method’s return type and all parameter types are @FlowSources(FlowSource.LITERAL) @FlowSinks(FlowSink.CONDITIONAL). As with all library annotations, it is trusted rather than checked. Thus, it should be used only if an external analysis has determined that it is correct for the annotated method, class, or package.
Annotation @ConservativeFlow expresses that each contained method should have the most conservative possible annotations: @FlowSources() @FlowSinks(ANY) on arguments, and @FlowSources(ANY) @FlowSinks() on return values. This is so conservative that it is sure to cause a type-checking failure whenever the method is used. When the analyst encounters such type-checking errors, the analyst can annotate the methods more appropriately. This is a way of knowing when a program uses a previously-unannotated library.
Annotation @PolyFlow expresses that each contained method should be annotated as @PolyFlowSource @PolyFlowSink for both the return types and all parameters.
By default, the flow checker is unsound. After getting the basic checks to pass, the stricter checks should be enabled, by running ant -Dsound=true flowtest. This two-phase approach was chosen to reduce the annotation effortand to give two separate phases of the annotation effort. The sound checking enforces invariant array subtyping and type safety in downcasts.
When strict checks are turned on, a cast (Object []) x, were x is of type Object, will result in a compiler warning:
[jsr308.javac] ... warning: "@FlowSinks @FlowSources(FlowSource.ANY) Object"
may not be casted to the type "@FlowSinks @FlowSources Object"
The reason is that there is not way for the type-checker to verify the component type of the array. There is no static knowledge about the actual runtime values in the array and important flow could be hidden. The analyst should argue why the downcast is safe.
Note that the main qualifier of a cast is automatically flow-refined by the cast expression.
Stricter checking also enforces invariant array subtyping, which is needed for sound array behavior in the absence of runtime checks. Flow inference automatically refines the type of array creation expressions depending on the left-hand side.
Methods like equals() and toString() that are inherited from Object are the most general possible, so that overriding methods can restrict the annotations further. Thus, they return FlowSource.ANY and no flow sinks.
Binary operations like string concatenation or integer addition, result in the least upper bound of the two component types.
This chapter describes best practices for annotating an Android application. In general, only fields and methods signatures in your own code and in libraries need to be annotated. Usually method bodies do not need to be annotated.
An application is fully annotated when the ant flowtest returns no errors. So the typical work flow is to run ant flowtest, add a few annotations to your own code or to API methods, and then run ant flowtest again. Repeat this until there are no more errors.
Typically, return types should be annotated with just FlowSources so that the FlowSinks can be inferred from the policy file as explained in Section 4.2. Similarly, parameters should only be annotated with FlowSinks, so that the FlowSources can be inferred from the policy file. Local variables should not have to be annotated, because their types can be inferred. Fields must be annotated with FlowSinks or FlowSources, or sometimes both. The guidelines for annotating API methods described in the next sections also apply to annotating methods in your application.
Stubfiles in sparta-code/src/sparta/checkers/flowstubfiles provides Android and Java library annotations. You may need to enhance it, if you find that your application uses APIs that are not yet annotated. After changing a stub file, remember to rebuild sparta.jar (ant jar). Also, the Checker Framework manual has more information on stub files. http://types.cs.washington.edu/checker-framework/current/checkers-manual.html#annotating-libraries
For every API method used by this app (i.e., those output by ant reportapiusage) that does not already appear in a stub file, read the Javadoc and decide what flow properties the method has. Use this information to annotate the method. The next few sections have guidelines on how to annotate certain kinds of methods.
To speed up annotating an entire API class, copy the output from Checker Framework’s StubGenerator to the correct stub file and then annotate the methods. See http://types.cs.washington.edu/checker-framework/current/checkers-manual.html#stub for more details.
If you read the javadoc and the method returns an object from a certain source, then you should annotated the return with that flow source and FlowSink.ANY. (If @FlowSinks is omitted then it is assumed to be empty which means that the object returned cannot be assigned to a variable with a sink.)
The getParameters method is an example of a method that returns an object with a source.
package android.hardware;
class Camera{
@FlowSources(FlowSource.CAMERA_SETTINGS) @FlowSinks(FlowSink.ANY)
Camera.Parameters getParameters ();
void setParameters (@FlowSources(FlowSource.ANY) @FlowSinks(CAMERA_SETTINGS)
Camera.Parameters params);
}
Camera.Parameters parameters
= mCamera.getParameters();
//Change some parameters
//If the policy file contains: CAMERA_SETTINGS->CAMERA_SETTINGS
//Then the following statement will not give an error
mCamera.setParameters(parameters);
If you read the javadoc for an API method an it sends some parameters to a FlowSink, then the parameters should be annotated with that flow sink and ANY flow source.
Example annotation
package android.database.sqlite;
class SQLiteDatabase{
public void execSQL (@FlowSinks(FlowSink.DATABASE) @FlowSource(FlowSource.ANY) String sql);
}
Use of the API
@FlowSources(SMS,LITERAL) String getSMSQuery(){..}
String mes = getSMSQuery();
//if SMS,LITERAL->DATABASE is in flow policy
//Then the following statement will not give an error
db.execSQL(mes);
The Android API frequently uses callbacks. These are methods that the developer must implement and register. In stub files, these callbacks should be annotated with source information and FlowSink.ANY.
An example annotation of a callback method
package android.hardware;
class Camera$PictureCallback{
//data: a byte array of the picture data
void onPictureTaken
(@FlowSource(CAMERA) @FlowSinks(FlowSink.ANY) byte[] data,
@FlowSource(CAMERA) @FlowSinks(FlowSink.ANY) Camera camera);
}
An example implementation of a callback
public void onPictureTaken
(@FlowSource(CAMERA) byte[] data, @FlowSource(CAMERA) Camera camera){
//If CAMERA->FILE_SYSTEM is in policy file
//Then the following statement will not give an error
writeToFile(data);
}
Some methods take the arguments passed, transform them, and then return them. These sorts of methods should be annotated with @PolyFlowSources @PloyFlowSinks to preserve the flow information. The declaration annotation @PolyFlow can be used instead of annotating all the parameters and return types. See Section 4.10.3 for more information
Math.min(...) is a good example of these kinds of methods.
package java.lang;
class Math{
@PolyFlow
int min(int i1, int i2);
}
Example use of @PolyFlow.
@FlowSources(FlowSource.LOCATION) int i1 = getLocation();
@FlowSources(FlowSource.NETWORK) int i2 = getLocationForNetwork();
@FlowSources({FlowSource.LOCATION,FlowSource.NETWORK)}) int min = Math.min(i1,i2);
This section explains four of the errors you are likely to see when annotated your application. This section also gives advice about how you might correct the error.
Every source-sink pair in your code must be listed in the flow policy or else a forbidden flow error will occur. To correct a forbidden flow error, add the forbidden flow to the policy file.
For example, fix the error below by adding LITERAL -> FILESYSTEM to the policy file.
NewTest.java:43: error: flow forbidden by flow-policy
test = new @FlowSinks(FlowSink.FILESYSTEM)@FlowSources(FlowSource.LITERAL) TestClass2(fs);
^
found: @FlowSinks(FlowSink.FILESYSTEM) @FlowSources(FlowSource.LITERAL) TestClass2
forbidden flows:
LITERAL -> FILESYSTEM
The most common errors are incompatible types. They can be in arguments, assignment, return, etc.
APIs that have not been annotated have been typed so conservatively that they will always produce incompatible types errors where the required is @FlowSinks(ANY) @FlowSources() or @FlowSinks() @FlowSources(ANY). These errors can be fixed by annotating the API method; Section 5.3 gives guidelines for annotating APIs. Below is an example of this sort of error.
HelloWorld.java:84: error: incompatible types in argument
.replace(R.id.container, fragment)
^
found : @FlowSinks(CONDITIONAL) @FlowSources(LITERAL) Fragment
required: @FlowSinks(ANY) @FlowSources({}) Fragment
If the incompatible types error is not from conservative defaulting, then the error must be fixed by adding or removing annotations in the application. For example, the error below can be fixed by adding ACCELEROMETER to the FlowSource of the return type.
HelloWorld.java:49: error: incompatible types in return.
return x;
^
found : @FlowSinks(CONDITIONAL) @FlowSources({LITERAL, ACCELEROMETER}) int
required: @FlowSinks(CONDITIONAL) @FlowSources(LITERAL) int
As explained in Section 4.5, any item in a conditional statement must have CONDITIONAL listed as a FlowSink. If a variable is only annotated with FlowSources and strict conditionals are not used, then CONDITIONAL is added as a flow sink by default.
For example, if input is a parameter in a method and is annotated with @FlowSinks(FlowSink.NETWORK), the following error will occur. To fix the error, add CONDITIONAL to the flow sink annotation.
HelloWorld.java:48: error: Conditions are not allowed to depend on flow information.
if(i1 > 2){
^
APIs are annotated in stub files. If a stub has a typo or the API method does not exist in the version of Android used.
For example, the error below can be fixed by removing the extra L in the method name.
StubParser: Method isLLowerCase(char) not found in type java.lang.Character
This error can be corrected by removing the extra L in the method name.
The method enableForgroundNdefPush(...) is not defined in android.nfc.NfcAdapter, so to fix the error below, this method should be removed from the stub file.
StubParser: Method enableForegroundNdefPush(Activity,NdefPushCallback)
not found in type android.nfc.NfcAdapter
This is a brief guide on how to analyze an Android application and find security vulnerabilities. This document gives a high-level view of the process, including how various tools relate to one another.
For details about how to install and run the code analysis tools, see Chapter 2.
Analyzing an annotated app is much simpler than analyzing an unannotated one. Basically, the analysis consists of answering affirmatively three questions.
Does the Flow Checker produce any errors or warnings? A properly annotated app should not produce any warnings or errors. If it does, this would be grounds for rejection. (You can follow the instructions in subsection 6.2.2 to set up the app for use with the Flow Checker and then run ant flowtest.)
Does the flow-policy file match the application description? There should not be any flows that are not explained in the description. These flows may be explicitly stated, such as “encrypt and sign messages, send them via your preferred email app.” Or a flow may only be implied, such as “This Application allows the user to share pics with their contacts.” In the first example, you would expect an EMAIL sink to appear somewhere in the policy file. In the second, “share” could mean a you would see a Flow Sink of EMAIL, SMS, NETWORK, or something else. Flows that are only implied in the description could be grounds for rejection if the description is too vague.
Does the justification for every @SuppressWarning make sense? Search for every instance of @SuppressWarnings("flow") and read the justification comment. Compare the justification to the actual code and determine if it make sense and should be allowed. No justification comment could be grounds for rejection.
The recommended work flow is:
The tools should be run in the following order.
The rest of this chapter gives more details about each step in the analysis process.
Checking applications
It is required to set three environment variables:
The SPARTA project needs to be updated to your development environment, so that the path to the Android SDK is correct and the correct version of Android is use. (More details about updating a project are available here: http://developer.android.com/tools/projects/projects-cmdline.html#UpdatingAProject.)To update the project with your Android settings run the following:
ant -buildfile $SPARTA_CODE/build.local.xml
Alternatively, you can run:
$ANDROID_HOME/tools/android update project --path . --target android-15
Edit the build.xml file of the project under analysis to add the
SPARTA build targets at the end, right before </project>:
<property environment="env"/>
<dirname property="checkers_dir" file="${env.CHECKERS}"/>
<basename property="checkers_base" file="${env.CHECKERS}"/>
<dirname property="sparta-code_dir" file="${env.SPARTA_CODE}"/>
<basename property="sparta-code_base" file="${env.SPARTA_CODE}"/>
<import file="${sparta-code_dir}/${sparta-code_base}/build.include.xml" optional="true"/>
To use Eclipse to look at and build the code, perform these simple steps:
Most Android apps will rely on an auto-generated R.java file in the /gen directory of the project. This will only be generated if there are no errors in the project. There may be errors in the resources (.../res directory) that could cause R.java to not be generated.
Additionally, if the app depends on an external .jar file (often located in the lib/ directory), it will compile in Eclipse but not with Ant. To fix this, in ant.properties, add “jar.libs.dir=lib” (or wherever the .jar is located).
Read the description of the application. Look at the AndroidManifest.xml file and:
Use your understanding of the App to write a flow-policy file. A full description of Flow Policies can be found in Section 4.3. You should also familiarize yourself with all the possible sources and sinks; all FlowSources are members of the enum sparta.checkers.quals.FlowSources.FlowSource and all FlowSinks are members of the enum sparta.checkers.quals.FlowSinks.FlowSink.
To begin, read the App description, looking for clues about the information flow. For example, if this is a map app, does the description say anything about sending your location data over the network? If so, then you should add LOCATION → NETWORK to the flow-policy file. Where else does the description say LOCATION data can go?
Theoretically you should be able to write a complete Flow Policy from the description if the App does not contain malware. In practice, you will have to add flows to the policy file as you more fully annotate the app, but you should make note of what additional flows you had to add.
After you have written the flows that are apparent from the description, you may want to see if the App has any “use permissions” in the AndroidManifest.xml file that are not listed in the flow policy. (Permissions are not one-to-one with sinks and sources, but the name of the permission should give a clue about which sources and sinks are involved.) For example, if the SEND_SMS permission is requested, then SMS should be listed as a flow sink somewhere in the policy file. If this is not the case, do not add it to the policy file, but pay close attention to how this permission is used in the code.
Run ant reportsuspicious to get a list of the most suspicious code locations. This target only reports about suspicious APIs that appear in the file: sparta-code/src/sparta/checkers/suspicious.astub. This report provides the most suspicious code locations and is intended to help localizing malware in an unannotated application.
The following example from the suspicious.astub file reports all calls
of the invoke method and, additionally, all constructor calls
of the class java.util.Random:
package java.lang.reflect;
class Method {
@ReportCall
public Object invoke(Object obj, Object [] objs) {}
}
package java.util;
@ReportCreation
class Random {}
In addition to running the report checker based on the
suspicious.astub file, the reportsuspicious target executes
the script sparta-code/suspicious.pl, which recursively searches for
suspicious String patterns (i.e., URIs, or IP and MAC addresses)
in .java and strings.xml files.
Generally, the sparta-code/suspicious.pl script takes two arguments:
For each match, the script reports the file name, line number, and the found pattern.
The suspicious.astub and suspicious.pl files should be enhanced with additional API uses and String patterns that turn out to be malicious for every analyzed application.
Run ant reqperms to see a list of the app’s methods that use calls that require certain Android permissions. You can use this to gain an understanding of where sensitive information may come from/go to in the application. This command produces errors like the following:
error: Call additionally requires permissions [android.permission.INTERNET], but caller only provides []!
You can remove the error by writing @RequiredPermissions(android.Manifest.permission.PERMISSION) in front of the method header in the source code.
Once all methods in the subject application are correctly annotated, ant reqperms will not issue any warnings. Grep for @RequiredPermissions to find the required permissions in the application.
Any permission that is required should already be listed in the AndroidManifest.xml file.
Run ant reportapiusage to get a report of the used APIs. The reportapiusage target only reports about APIs that appear in the file: sparta-code/src/sparta/checkers/apiusage.astub. This report is useful for program comprehension but not intended to provide suspicious code locations. For suspicious API use, see 6.2.5.
The following example from apiusage.astub causes the corresponding
checker to report the use of all entities in the package
com.android:
@ReportUse package com.android;
The apiusage.astub file should be enhanced during and after the analysis of an application — add the entities that are most crucial to understand the behaviour of an application.
Stub files in sparta-code/src/sparta/checkers/flowstubfiles provides Android and Java library annotations. You may need to enhance it, if you find that your application uses APIs that are not yet annotated. (The more APIs get annotated, the less work you will have to do in this step for each new app.) You should not annotate libraries that are unique to your application in flowstubfiles; instead, annotated them in a new *.astub file. You can pass this file to be used with flowtest or any of the other ant targets.
ant -Dstubs=path/myAnnoLib.astub flowtest
An example:
package android.telephony;
class TelephonyManager {
public @FlowSources(FlowSource.PHONE_NUMBER) String getLine1Number();
public @FlowSources(FlowSource.IMEI) String getDeviceId();
}
The above annotates two methods in class TelephonyManager. It indicates that the getLine1Number function returns a String which is a phone number. For more examples, look into the stub files. Also, see the manual http://types.cs.washington.edu/checker-framework/current/checkers-manual.html#annotating-libraries
For every API method used by this app (i.e., those output by ant reportapiusage) that does not already appear a stub file do the following.
If according to the description, the method has no flow, then the parameters and return type should be annotated with @PolyFlowSources @PloyFlowSinks to preserve the flow information.
Note: you have not yet added any annotations to the app itself.
You do not have to run this step — you can skip it if you prefer.
Run ant flowshow to get a report of the existing flow. For every type use in the application, it indicates the flow sources and sinks for that variable. This is exactly the annotations written in the program, plus possibly some additional annotations that are inferred by the Checker Framework.
This step does not perform type-checking; it only visualizes the flow information written in the program or libraries as annotations, or inferred from those annotations.
For an unannotated program, the report will not be informative: it only contains API annotations that are propagated to local variables. The report will become more informative as you add more and more annotations to the application. So, you can periodically rerun this step.
Run ant flowtest on the application. Eliminate each warning in one of two ways.
A prime example is a String literal that should be allowed to be sent over the network. By default, every literal has @FlowSinks() (i.e., nothing) and @FlowSources().
@SuppressWarnings("flow") // manually verified to not contain secret data
@FlowSinks(FlowSink.NETWORK) String url = "http://bazinga.com/";
Without suppression the assignment raises an error, because string literals are assumed to be annotated with FlowSources(FlowSource.LITERAL). By adding the suppression, you assert that it’s OK to send this string to the network.
Focus on the most interesting flow sources and try to connect the flow sources and sinks in the application. Instead of trying to completely annotate only the sources or only the sinks, skim over all the reports and use your intuition to decide which parts of the application to focus on. Try to focus on the parts with the (most) connections between sources and sinks.
Most types will only use either a @FlowSources or @FlowSinks annotation. The goal is to find places where you need both annotations, e.g. to express that information that comes from the camera may go to the network:
@FlowSources(FlowSource.CAMERA)
@FlowSinks(@FlowSink.NETWORK) Picture data;
Such a type connects sources and sinks and one needs to carefully decide whether this is a desired information flow or not.
You can continue to use ant flowshow to visualize the annotation progress.
Once all warnings were resolved, run ant -Dstricter=true flowtest on the application. Providing the stricter option enables additional checks that are required for soundness, but would be disruptive to enable initially. In particular, the tests for casts and array subtyping are stricter. See the discussion in Chapter 4.
This option will also use the stricter conditional rule. (LITERAL → CONDITIONAL rather than the relaxed ANY → CONDITIONAL)
The goal of an application developer is to create a safe, functional application — and to write the documentation and code so that the safety and functionality are immediately obvious. In particular, the code and documentation should be clear and complete, and the system should pass all the tests that the SPARTA toolset performs. If the application developer fails to meet any of these objectives, then the application will be rejected from the app store, and the fault will be with the application developer, not with the app store.
A malicious developer would need to write clear documentation and code, but would attempt to hide malicious behavior in the app nonetheless. If the documentation or code is not clear, or if the malicious behavior is not well-hidden, or if the SPARTA tools do not confirm that the code conforms to the documentation, then the malicious developer has failed in his task.
Note that the malicious developer’s goal is more difficult than just writing malicious code, and is even more difficult than writing well-hidden malicious code. The reason is that the SPARTA toolset encourages good coding style: poor style requires more warning suppressions. The SPARTA tools lead a programmer to better, clearer code.
Here are some specific requirements of the app developer:
Provide source code. Provide a build file (for Ant, Maven, Android, etc.).
The English description should include how the information flows between parts of the application (the paths along which information flows), and the conditions under which it flows (such as only after a particular user action or external trigger). These will eventually be represented in the SPARTA toolset’s file format and checked by SPARTA, but they are not yet.
If rewriting the code is impossible, then every remaining warning should be suppressed with a @SuppressWarnings annotation. Every @SuppressWarnings annotation requires a clear, compelling justification regarding why the code is actually correct and safe (even though the type-checker cannot prove this property), and why the code cannot be rewritten to address the warning. This justification should be written in the source code at the location of the @SuppressWarnings annotation.
An excessive number of type-checker warnings, or missing justifications for warning suppressions, is grounds for rejection from the app store.
This chapter is currently disorganized notes that will be incorporated into either this manual or the Checker Framework manual.
[Note: FlowSources(…) is shorthand noting that the specific flow properties aren’t relevant.]
JSR 308 extends the Java language to allow annotations in more locations. Java 8 will include support for this extended syntax. The Checker Framework builds on a version of OpenJDK that already supports this syntax. However, existing compilers do not support this syntax and will raise an error. This is an issue in particular if you want to analyze applications in Eclipse.
Eclipse accepts annotations only in the locations supported by Java 1.5, that is, only declaration locations; examples are fields, local variables, parameters, and methods (return type annotations go in the same place).
Eclipse won’t accept annotations in locations that were added in JSR 308; added locations include type arguments, object creation, casts, type parameter bounds, and others. To avoid syntax errors from Eclipse or other Java compilers, you need to put such annotations in comments. The SPARTA tools will interpret them, but Eclipse and other Java compilers will ignore them.
For details, see section “Annotations in comments” in the Checker Framework manual.
Sometimes method type argument inference does not interact well with type qualifiers. In such situations, you might need to provide explicit method type arguments, for which the syntax is as follows:
Collections.</*@FlowSources(...)*/ Object>sort(l, c);
This document contains details that are only relevant for people inside the SPARTA team at UW.
The SPARTA team uses four Mercurial repositories: sparta-code for the source code of the SPARTA toolset, sparta-subjects for test applications (case studies), sparta-meetings for notes about UW team meetings, and apac-meetings for notes about DARPA meetings.
To get a copy do:
hg clone https://dada.cs.washington.edu/hgweb/sparta-code
and similarly for the other three repositories.
To push your changes to the repository you need to be in the sparta Unix group. Contact Werner or Mike to get the permission. Also, if you are going to push changes, please add a .hgrc file to your home directory on the server. The .hgrc file should contain:
[trusted] users = wmdietl groups = sparta
This allows emails to be sent when you push changes.
Note that SPARTA as well as the Checker Framework are evolving rapidly. Thus you should periodically get the latest version of the source code (by running hg fetch) and rebuild the projects.
After installing your copy, try to run ant in sparta-subjects/Sky:
$ ant flowtest
If it gives results like this, you’re ready to work on annotating!
[jsr308.javac] .../sparta-subjects/Sky/src/org/jsharkey/sky/WebserviceHelper.java:308: error: incompatible types. [jsr308.javac] HttpGet request = new HttpGet(String.format(WEBSERVICE_URL, lat, lon, days)); [jsr308.javac] ^ [jsr308.javac] found : @FlowSinks @FlowSources String [jsr308.javac] required: @sparta.checkers.quals.FlowSinks(sparta.checkers.quals.FlowSinks.FlowSink.NETWORK) @FlowSources String
If you want to add a new application, put it under the sparta-subjects directory.
You may need to get Android source code to get sense of what API returns (or gets) what type of data. See http://source.android.com/source/index.html You can find the list of all APIs from the android source code in frameworks/base/api/15.txt - api list for api version 15 (Android 4.0.3) Accessing resource is closely related to android permissions (some of the resources are not protected with permissions though). Android permission list is at: http://developer.android.com/reference/android/Manifest.permission.html Hints to add annotations could be permissionmap (which permission is required to call which functions): http://www.android-permissions.org/permissionmap.html