Analyzing data flow in C and C++¶
You can use data flow analysis to track the flow of potentially malicious or insecure data that can cause vulnerabilities in your codebase.
About this article¶
This article describes how data flow analysis is implemented in the CodeQL libraries for C/C++ and includes examples to help you write your own data flow queries. The following sections describe how to use the libraries for local data flow, global data flow, and taint tracking.
Note
The modular API for data flow described here is available from CodeQL 2.13.0. The legacy library is deprecated and will be removed in December 2024. For information about how the library has changed and how to migrate any existing queries to the modular API, see New dataflow API for CodeQL query writing.
About data flow¶
Data flow analysis computes the possible values that a variable can hold at various points in a program, determining how those values propagate through the program, and where they are used. In CodeQL, you can model both local data flow and global data flow. For a more general introduction to modeling data flow, see “About data flow analysis.”
Local data flow¶
Local data flow is data flow within a single function. Local data flow is usually easier, faster, and more precise than global data flow, and is sufficient for many queries.
Using local data flow¶
The local data flow library is in the module DataFlow, which defines the class Node denoting any element that data can flow through. Nodes are divided into expression nodes (ExprNode, IndirectExprNode) and parameter nodes (ParameterNode, IndirectParameterNode). The indirect nodes represent expressions or parameters after a fixed number of pointer dereferences.
It is possible to map between data flow nodes and expressions or parameters using the member predicates asExpr, asIndirectExpr, and asParameter:
class Node {
/**
* Gets the expression corresponding to this node, if any.
*/
Expr asExpr() { ... }
/**
* Gets the expression corresponding to a node that is obtained after dereferencing
* the expression `index` times, if any.
*/
Expr asIndirectExpr(int index) { ... }
/**
* Gets the parameter corresponding to this node, if any.
*/
Parameter asParameter() { ... }
/**
* Gets the parameter corresponding to a node that is obtained after dereferencing
* the parameter `index` times.
*/
Parameter asParameter(int index) { ... }
...
}
The predicate localFlowStep(Node nodeFrom, Node nodeTo) holds if there is an immediate data flow edge from the node nodeFrom to the node nodeTo. The predicate can be applied recursively (using the + and * operators), or through the predefined recursive predicate localFlow, which is equivalent to localFlowStep*.
For example, finding flow from a parameter source to an expression sink in zero or more local steps can be achieved as follows, where nodeFrom and nodeTo are of type DataFlow::Node:
nodeFrom.asParameter() = source and
nodeTo.asExpr() = sink and
DataFlow::localFlow(nodeFrom, nodeTo)
Using local taint tracking¶
Local taint tracking extends local data flow by including non-value-preserving flow steps. For example:
int i = tainted_user_input();
some_big_struct *array = malloc(i * sizeof(some_big_struct));
In this case, the argument to malloc is tainted.
The local taint tracking library is in the module TaintTracking. Like local data flow, a predicate localTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) holds if there is an immediate taint propagation edge from the node nodeFrom to the node nodeTo. The predicate can be applied recursively (using the + and * operators), or through the predefined recursive predicate localTaint, which is equivalent to localTaintStep*.
For example, finding taint propagation from a parameter source to an expression sink in zero or more local steps can be achieved as follows, where nodeFrom and nodeTo are of type DataFlow::Node:
nodeFrom.asParameter() = source and
nodeTo.asExpr() = sink and
TaintTracking::localTaint(nodeFrom, nodeTo)
Examples¶
The following query finds the filename passed to fopen:
import cpp
from Function fopen, FunctionCall fc
where
fopen.hasGlobalName("fopen") and
fc.getTarget() = fopen
select fc.getArgument(0)
However, this will only give the expression in the argument, not the values which could be passed to it. Instead we can use local data flow to find all expressions that flow into the argument, where we use asIndirectExpr(1). This is because we are interested in the value of the string passed to fopen, not the pointer pointing to it:
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
from Function fopen, FunctionCall fc, Expr src, DataFlow::Node source, DataFlow::Node sink
where
fopen.hasGlobalName("fopen") and
fc.getTarget() = fopen and
source.asIndirectExpr(1) = src and
sink.asIndirectExpr(1) = fc.getArgument(0) and
DataFlow::localFlow(source, sink)
select src
Then we can vary the source and, for example, use the parameter of a function. The following query finds where a parameter is used when opening a file:
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
from Function fopen, FunctionCall fc, Parameter p, DataFlow::Node source, DataFlow::Node sink
where
fopen.hasGlobalName("fopen") and
fc.getTarget() = fopen and
source.asParameter(1) = p and
sink.asIndirectExpr(1) = fc.getArgument(0) and
DataFlow::localFlow(source, sink)
select p
The following example finds calls to formatting functions where the format string is not hard-coded.
import semmle.code.cpp.dataflow.new.DataFlow
import semmle.code.cpp.commons.Printf
from FormattingFunction format, FunctionCall call, Expr formatString, DataFlow::Node sink
where
call.getTarget() = format and
call.getArgument(format.getFormatParameterIndex()) = formatString and
sink.asIndirectExpr(1) = formatString and
not exists(DataFlow::Node source |
DataFlow::localFlow(source, sink) and
source.asIndirectExpr(1) instanceof StringLiteral
)
select call, "Argument to " + format.getQualifiedName() + " isn't hard-coded."
Global data flow¶
Global data flow tracks data flow throughout the entire program, and is therefore more powerful than local data flow. However, global data flow is less precise than local data flow, and the analysis typically requires significantly more time and memory to perform.
Note
You can model data flow paths in CodeQL by creating path queries. To view data flow paths generated by a path query in CodeQL for VS Code, you need to make sure that it has the correct metadata and
selectclause. For more information, see Creating path queries.
Using global data flow¶
The global data flow library is used by implementing the signature DataFlow::ConfigSig and applying the module DataFlow::Global<ConfigSig> as follows:
import semmle.code.cpp.dataflow.new.DataFlow
module MyFlowConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node source) {
...
}
predicate isSink(DataFlow::Node sink) {
...
}
}
module MyFlow = DataFlow::Global<MyFlowConfiguration>;
The following predicates are defined in the configuration:
isSource—defines where data may flow fromisSink—defines where data may flow toisBarrier—optional, restricts the data flowisAdditionalFlowStep—optional, adds additional flow steps
The data flow analysis is performed using the predicate flow(DataFlow::Node source, DataFlow::Node sink):
from DataFlow::Node source, DataFlow::Node sink
where MyFlow::flow(source, sink)
select source, "Data flow to $@.", sink, sink.toString()
Using global taint tracking¶
Global taint tracking is to global data flow as local taint tracking is to local data flow. That is, global taint tracking extends global data flow with additional non-value-preserving steps. The global taint tracking library is used by applying the module TaintTracking::Global<ConfigSig> to your configuration instead of DataFlow::Global<ConfigSig> as follows:
import semmle.code.cpp.dataflow.new.TaintTracking
module MyFlowConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node source) {
...
}
predicate isSink(DataFlow::Node sink) {
...
}
}
module MyFlow = TaintTracking::Global<MyFlowConfiguration>;
The resulting module has an identical signature to the one obtained from DataFlow::Global<ConfigSig>.
Examples¶
The following data flow configuration tracks data flow from environment variables to opening files in a Unix-like environment:
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
module EnvironmentToFileConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node source) {
exists(Function getenv |
source.asIndirectExpr(1).(FunctionCall).getTarget() = getenv and
getenv.hasGlobalName("getenv")
)
}
predicate isSink(DataFlow::Node sink) {
exists(FunctionCall fc |
sink.asIndirectExpr(1) = fc.getArgument(0) and
fc.getTarget().hasGlobalName("fopen")
)
}
}
module EnvironmentToFileFlow = DataFlow::Global<EnvironmentToFileConfiguration>;
from
Expr getenv, Expr fopen, DataFlow::Node source, DataFlow::Node sink
where
source.asIndirectExpr(1) = getenv and
sink.asIndirectExpr(1) = fopen and
EnvironmentToFileFlow::flow(source, sink)
select fopen, "This 'fopen' uses data from $@.", getenv, "call to 'getenv'"
The following taint-tracking configuration tracks data from a call to ntohl to an array index operation. It uses the Guards library to recognize expressions that have been bounds-checked, and defines isBarrier to prevent taint from propagating through them. It also uses isAdditionalFlowStep to add flow from loop bounds to loop indexes.
import cpp
import semmle.code.cpp.controlflow.Guards
import semmle.code.cpp.dataflow.new.TaintTracking
module NetworkToBufferSizeConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node node) {
node.asExpr().(FunctionCall).getTarget().hasGlobalName("ntohl")
}
predicate isSink(DataFlow::Node node) {
exists(ArrayExpr ae | node.asExpr() = ae.getArrayOffset())
}
predicate isAdditionalFlowStep(DataFlow::Node pred, DataFlow::Node succ) {
exists(Loop loop, LoopCounter lc |
loop = lc.getALoop() and
loop.getControllingExpr().(RelationalOperation).getGreaterOperand() = pred.asExpr()
|
succ.asExpr() = lc.getVariableAccessInLoop(loop)
)
}
predicate isBarrier(DataFlow::Node node) {
exists(GuardCondition gc, Variable v |
gc.getAChild*() = v.getAnAccess() and
node.asExpr() = v.getAnAccess() and
gc.controls(node.asExpr().getBasicBlock(), _) and
not exists(Loop loop | loop.getControllingExpr() = gc)
)
}
}
module NetworkToBufferSizeFlow = TaintTracking::Global<NetworkToBufferSizeConfiguration>;
from DataFlow::Node ntohl, DataFlow::Node offset
where NetworkToBufferSizeFlow::flow(ntohl, offset)
select offset, "This array offset may be influenced by $@.", ntohl,
"converted data from the network"
Exercises¶
Exercise 2: Write a query that finds all hard-coded strings used to create a host_ent via gethostbyname, using global data flow. (Answer)
Exercise 3: Write a class that represents flow sources from getenv. (Answer)
Exercise 4: Using the answers from 2 and 3, write a query which finds all global data flows from getenv to gethostbyname. (Answer)
Answers¶
Exercise 1¶
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
from StringLiteral sl, FunctionCall fc, DataFlow::Node source, DataFlow::Node sink
where
fc.getTarget().hasName("gethostbyname") and
source.asIndirectExpr(1) = sl and
sink.asIndirectExpr(1) = fc.getArgument(0) and
DataFlow::localFlow(source, sink)
select sl, fc
Exercise 2¶
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
module LiteralToGethostbynameConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node source) {
source.asIndirectExpr(1) instanceof StringLiteral
}
predicate isSink(DataFlow::Node sink) {
exists(FunctionCall fc |
sink.asIndirectExpr(1) = fc.getArgument(0) and
fc.getTarget().hasName("gethostbyname")
)
}
}
module LiteralToGethostbynameFlow = DataFlow::Global<LiteralToGethostbynameConfiguration>;
from
StringLiteral sl, FunctionCall fc, DataFlow::Node source, DataFlow::Node sink
where
source.asIndirectExpr(1) = sl and
sink.asIndirectExpr(1) = fc.getArgument(0) and
LiteralToGethostbynameFlow::flow(source, sink)
select sl, fc
Exercise 3¶
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
class GetenvSource extends DataFlow::Node {
GetenvSource() { this.asIndirectExpr(1).(FunctionCall).getTarget().hasGlobalName("getenv") }
}
Exercise 4¶
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
class GetenvSource extends DataFlow::Node {
GetenvSource() { this.asIndirectExpr(1).(FunctionCall).getTarget().hasGlobalName("getenv") }
}
module GetenvToGethostbynameConfiguration implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node source) { source instanceof GetenvSource }
predicate isSink(DataFlow::Node sink) {
exists(FunctionCall fc |
sink.asIndirectExpr(1) = fc.getArgument(0) and
fc.getTarget().hasName("gethostbyname")
)
}
}
module GetenvToGethostbynameFlow = DataFlow::Global<GetenvToGethostbynameConfiguration>;
from
Expr getenv, FunctionCall fc, DataFlow::Node source, DataFlow::Node sink
where
source.asIndirectExpr(1) = getenv and
sink.asIndirectExpr(1) = fc.getArgument(0) and
GetenvToGethostbynameFlow::flow(source, sink)
select getenv, fc
Further reading¶
- Exploring data flow with path queries in the GitHub documentation.