C++: Add a flow-after-free library.

Author: MathiasVP

cpp/ql/src/Critical/FlowAfterFree.qll

@@ -0,0 +1,113 @@
+import cpp
+import semmle.code.cpp.dataflow.new.DataFlow
+private import semmle.code.cpp.ir.IR
+
+/**
+ * Signature for a predicate that holds if `n.asExpr() = e` and `n` is a sink in
+ * the `FlowFromFreeConfig` module.
+ */
+private signature predicate isSinkSig(DataFlow::Node n, Expr e);
+
+/**
+ * Holds if `dealloc` is a deallocation expression and `e` is an expression such
+ * that `isFree(_, e)` holds for some `isFree` predicate satisfying `isSinkSig`,
+ * and this source-sink pair should be excluded from the analysis.
+ */
+bindingset[dealloc, e]
+private signature predicate isExcludedSig(DeallocationExpr dealloc, Expr e);
+
+/**
+ * Holds if `(b1, i1)` strictly post-dominates `(b2, i2)`
+ */
+bindingset[i1, i2]
+predicate strictlyPostDominates(IRBlock b1, int i1, IRBlock b2, int i2) {
+  b1 = b2 and
+  i1 > i2
+  or
+  b1.strictlyPostDominates(b2)
+}
+
+/**
+ * Holds if `(b1, i1)` strictly dominates `(b2, i2)`
+ */
+bindingset[i1, i2]
+predicate strictlyDominates(IRBlock b1, int i1, IRBlock b2, int i2) {
+  b1 = b2 and
+  i1 < i2
+  or
+  b1.strictlyDominates(b2)
+}
+
+/**
+ * Constructs a `FlowFromFreeConfig` module that can be used to find flow between
+ * a pointer being freed by some deallocation function, and a user-specified sink.
+ *
+ * In order to reduce false positives, the set of sinks is restricted to only those
+ * that satisfy at least one of the following two criteria:
+ * 1. The source dominates the sink, or
+ * 2. The sink post-dominates the source.
+ */
+module FlowFromFree<isSinkSig/2 isASink, isExcludedSig/2 isExcluded> {
+  module FlowFromFreeConfig implements DataFlow::StateConfigSig {
+    class FlowState instanceof Expr {
+      FlowState() { isFree(_, this, _) }
+
+      string toString() { result = super.toString() }
+    }
+
+    predicate isSource(DataFlow::Node node, FlowState state) { isFree(node, state, _) }
+
+    pragma[inline]
+    predicate isSink(DataFlow::Node sink, FlowState state) {
+      exists(
+        Expr e, DataFlow::Node source, IRBlock b1, int i1, IRBlock b2, int i2,
+        DeallocationExpr dealloc
+      |
+        isASink(sink, e) and
+        isFree(source, state, dealloc) and
+        e != state and
+        source.hasIndexInBlock(b1, i1) and
+        sink.hasIndexInBlock(b2, i2) and
+        not isExcluded(dealloc, e)
+      |
+        strictlyDominates(b1, i1, b2, i2)
+        or
+        strictlyPostDominates(b2, i2, b1, i1)
+      )
+    }
+
+    predicate isBarrierIn(DataFlow::Node n) {
+      n.asIndirectExpr() = any(AddressOfExpr aoe)
+      or
+      n.asIndirectExpr() = any(Call call).getAnArgument()
+      or
+      exists(Expr e |
+        n.asIndirectExpr() = e.(PointerDereferenceExpr).getOperand() or
+        n.asIndirectExpr() = e.(ArrayExpr).getArrayBase()
+      |
+        e = any(StoreInstruction store).getDestinationAddress().getUnconvertedResultExpression()
+      )
+    }
+
+    predicate isBarrier(DataFlow::Node n, FlowState state) { none() }
+
+    predicate isAdditionalFlowStep(
+      DataFlow::Node n1, FlowState state1, DataFlow::Node n2, FlowState state2
+    ) {
+      none()
+    }
+  }
+
+  import DataFlow::GlobalWithState<FlowFromFreeConfig>
+}
+
+/**
+ * Holds if `n` is a dataflow node such that `n.asExpr() = e` and `e`
+ * is being freed by a deallocation function `dealloc`.
+ */
+predicate isFree(DataFlow::Node n, Expr e, DeallocationExpr dealloc) {
+  e = dealloc.getFreedExpr() and
+  e = n.asExpr() and
+  // Ignore realloc functions
+  not exists(dealloc.(FunctionCall).getTarget().(AllocationFunction).getReallocPtrArg())
+}