C++: Add more barriers to cpp/overrun-write

cpp/ql/lib/change-notes/2025-07-23-overrun-write.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/overrun-write` query now recognizes more bound checks and thus produces fewer false positives.

cpp/ql/lib/semmle/code/cpp/security/InvalidPointerDereference/AllocationToInvalidPointer.qll

@@ -53,44 +53,12 @@
 
 private import cpp
 private import semmle.code.cpp.ir.dataflow.internal.ProductFlow
+private import semmle.code.cpp.security.ProductFlowUtils.ProductFlowUtils
 private import semmle.code.cpp.ir.ValueNumbering
 private import semmle.code.cpp.controlflow.IRGuards
 private import codeql.util.Unit
 private import semmle.code.cpp.rangeanalysis.new.RangeAnalysisUtil
 
-private VariableAccess getAVariableAccess(Expr e) { e.getAChild*() = result }
-
-/**
- * Gets a (sub)expression that may be the result of evaluating `size`.
- *
- * For example, `getASizeCandidate(a ? b : c)` gives `a ? b : c`, `b` and `c`.
- */
-bindingset[size]
-pragma[inline_late]
-private Expr getASizeCandidate(Expr size) {
-  result = size
-  or
-  result = [size.(ConditionalExpr).getThen(), size.(ConditionalExpr).getElse()]
-}
-
-/**
- * Holds if the `(n, state)` pair represents the source of flow for the size
- * expression associated with `alloc`.
- */
-predicate hasSize(HeuristicAllocationExpr alloc, DataFlow::Node n, int state) {
-  exists(VariableAccess va, Expr size, int delta, Expr s |
-    size = alloc.getSizeExpr() and
-    s = getASizeCandidate(size) and
-    // Get the unique variable in a size expression like `x` in `malloc(x + 1)`.
-    va = unique( | | getAVariableAccess(s)) and
-    // Compute `delta` as the constant difference between `x` and `x + 1`.
-    bounded1(any(Instruction instr | instr.getUnconvertedResultExpression() = s),
-      any(LoadInstruction load | load.getUnconvertedResultExpression() = va), delta) and
-    n.asExpr() = va and
-    state = delta
-  )
-}
-
 /**
  * Gets the virtual dispatch branching limit when calculating field flow while searching
  * for flow from an allocation to the construction of an out-of-bounds pointer.
@@ -100,125 +68,6 @@ predicate hasSize(HeuristicAllocationExpr alloc, DataFlow::Node n, int state) {
  */
 int allocationToInvalidPointerFieldFlowBranchLimit() { result = 0 }
 
-/**
- * A module that encapsulates a barrier guard to remove false positives from flow like:
- * ```cpp
- * char *p = new char[size];
- * // ...
- * unsigned n = size;
- * // ...
- * if(n < size) {
- *   use(*p[n]);
- * }
- * ```
- * In this case, the sink pair identified by the product flow library (without any additional barriers)
- * would be `(p, n)` (where `n` is the `n` in `p[n]`), because there exists a pointer-arithmetic
- * instruction `pai = a + b` such that:
- * 1. the allocation flows to `a`, and
- * 2. `b <= n` where `n` is the `n` in `p[n]`
- * but because there's a strict comparison that compares `n` against the size of the allocation this
- * snippet is fine.
- */
-private module SizeBarrier {
-  private module SizeBarrierConfig implements DataFlow::ConfigSig {
-    predicate isSource(DataFlow::Node source) {
-      // The sources is the same as in the sources for the second
-      // projection in the `AllocToInvalidPointerConfig` module.
-      hasSize(_, source, _) and
-      InterestingPointerAddInstruction::isInterestingSize(source)
-    }
-
-    int fieldFlowBranchLimit() { result = allocationToInvalidPointerFieldFlowBranchLimit() }
-
-    /**
-     * Holds if `small <= large + k` holds if `g` evaluates to `testIsTrue`.
-     */
-    additional predicate isSink(
-      DataFlow::Node small, DataFlow::Node large, IRGuardCondition g, int k, boolean testIsTrue
-    ) {
-      // The sink is any "large" side of a relational comparison. i.e., the `large` expression
-      // in a guard such as `small <= large + k`.
-      g.comparesLt(small.asOperand(), large.asOperand(), k + 1, true, testIsTrue)
-    }
-
-    predicate isSink(DataFlow::Node sink) { isSink(_, sink, _, _, _) }
-  }
-
-  module SizeBarrierFlow = DataFlow::Global<SizeBarrierConfig>;
-
-  private int getASizeAddend(DataFlow::Node node) {
-    exists(DataFlow::Node source |
-      SizeBarrierFlow::flow(source, node) and
-      hasSize(_, source, result)
-    )
-  }
-
-  /**
-   * Holds if `small <= large + k` holds if `g` evaluates to `edge`.
-   */
-  private predicate operandGuardChecks(
-    IRGuardCondition g, Operand small, DataFlow::Node large, int k, boolean edge
-  ) {
-    SizeBarrierFlow::flowTo(large) and
-    SizeBarrierConfig::isSink(DataFlow::operandNode(small), large, g, k, edge)
-  }
-
-  /**
-   * Gets an instruction `instr` that is guarded by a check such as `instr <= small + delta` where
-   * `small <= _ + k` and `small` is the "small side" of of a relational comparison that checks
-   * whether `small <= size` where `size` is the size of an allocation.
-   */
-  Instruction getABarrierInstruction0(int delta, int k) {
-    exists(
-      IRGuardCondition g, ValueNumber value, Operand small, boolean edge, DataFlow::Node large
-    |
-      // We know:
-      // 1. result <= value + delta (by `bounded`)
-      // 2. value <= large + k (by `operandGuardChecks`).
-      // So:
-      // result <= value + delta (by 1.)
-      //        <= large + k + delta (by 2.)
-      small = value.getAUse() and
-      operandGuardChecks(pragma[only_bind_into](g), pragma[only_bind_into](small), large,
-        pragma[only_bind_into](k), pragma[only_bind_into](edge)) and
-      bounded(result, value.getAnInstruction(), delta) and
-      g.controls(result.getBlock(), edge) and
-      k < getASizeAddend(large)
-    )
-  }
-
-  /**
-   * Gets an instruction that is guarded by a guard condition which ensures that
-   * the value of the instruction is upper-bounded by size of some allocation.
-   */
-  bindingset[state]
-  pragma[inline_late]
-  Instruction getABarrierInstruction(int state) {
-    exists(int delta, int k |
-      state > k + delta and
-      // result <= "size of allocation" + delta + k
-      //        < "size of allocation" + state
-      result = getABarrierInstruction0(delta, k)
-    )
-  }
-
-  /**
-   * Gets a `DataFlow::Node` that is guarded by a guard condition which ensures that
-   * the value of the node is upper-bounded by size of some allocation.
-   */
-  DataFlow::Node getABarrierNode(int state) {
-    exists(DataFlow::Node source, int delta, int k |
-      SizeBarrierFlow::flow(source, result) and
-      hasSize(_, source, state) and
-      result.asInstruction() = SizeBarrier::getABarrierInstruction0(delta, k) and
-      state > k + delta
-      // so now we have:
-      // result <= "size of allocation" + delta + k
-      //        < "size of allocation" + state
-    )
-  }
-}
-
 private module InterestingPointerAddInstruction {
   private module PointerAddInstructionConfig implements DataFlow::ConfigSig {
     predicate isSource(DataFlow::Node source) {
@@ -227,7 +76,7 @@ private module InterestingPointerAddInstruction {
       hasSize(source.asExpr(), _, _)
     }
 
-    int fieldFlowBranchLimit() { result = allocationToInvalidPointerFieldFlowBranchLimit() }
+    predicate fieldFlowBranchLimit = allocationToInvalidPointerFieldFlowBranchLimit/0;
 
     predicate isSink(DataFlow::Node sink) {
       sink.asInstruction() = any(PointerAddInstruction pai).getLeft()
@@ -263,6 +112,17 @@ private module InterestingPointerAddInstruction {
   }
 }
 
+private module SizeBarrierInput implements SizeBarrierInputSig {
+  predicate fieldFlowBranchLimit = allocationToInvalidPointerFieldFlowBranchLimit/0;
+
+  predicate isSource(DataFlow::Node source) {
+    // The sources is the same as in the sources for the second
+    // projection in the `AllocToInvalidPointerConfig` module.
+    hasSize(_, source, _) and
+    InterestingPointerAddInstruction::isInterestingSize(source)
+  }
+}
+
 /**
  * A product-flow configuration for flow from an `(allocation, size)` pair to a
  * pointer-arithmetic operation `pai` such that `pai <= allocation + size`.
@@ -301,7 +161,7 @@ private module Config implements ProductFlow::StateConfigSig {
   private import semmle.code.cpp.ir.dataflow.internal.DataFlowPrivate
 
   predicate isBarrier2(DataFlow::Node node, FlowState2 state) {
-    node = SizeBarrier::getABarrierNode(state)
+    node = SizeBarrier<SizeBarrierInput>::getABarrierNode(state)
   }
 
   predicate isBarrier2(DataFlow::Node node) {
@@ -357,8 +217,8 @@ private predicate pointerAddInstructionHasBounds0(
     sizeInstr = sizeSink.asInstruction() and
     // pai.getRight() <= sizeSink + delta
     bounded1(right, sizeInstr, delta) and
-    not right = SizeBarrier::getABarrierInstruction(delta) and
-    not sizeInstr = SizeBarrier::getABarrierInstruction(delta)
+    not right = SizeBarrier<SizeBarrierInput>::getABarrierInstruction(delta) and
+    not sizeInstr = SizeBarrier<SizeBarrierInput>::getABarrierInstruction(delta)
   )
 }
 

cpp/ql/lib/semmle/code/cpp/security/ProductFlowUtils/ProductFlowUtils.qll

@@ -0,0 +1,167 @@
+/**
+ * This file provides the `SizeBarrier` module which provides barriers for
+ * both the `cpp/invalid-pointer-deref` query and the `cpp/overrun-write`
+ * query.
+ */
+
+private import cpp
+private import semmle.code.cpp.dataflow.new.DataFlow
+private import semmle.code.cpp.ir.ValueNumbering
+private import semmle.code.cpp.controlflow.IRGuards
+private import semmle.code.cpp.rangeanalysis.new.RangeAnalysisUtil
+
+private VariableAccess getAVariableAccess(Expr e) { e.getAChild*() = result }
+
+/**
+ * Gets a (sub)expression that may be the result of evaluating `size`.
+ *
+ * For example, `getASizeCandidate(a ? b : c)` gives `a ? b : c`, `b` and `c`.
+ */
+bindingset[size]
+pragma[inline_late]
+private Expr getASizeCandidate(Expr size) {
+  result = size
+  or
+  result = [size.(ConditionalExpr).getThen(), size.(ConditionalExpr).getElse()]
+}
+
+/**
+ * Holds if the `(n, state)` pair represents the source of flow for the size
+ * expression associated with `alloc`.
+ */
+predicate hasSize(HeuristicAllocationExpr alloc, DataFlow::Node n, int state) {
+  exists(VariableAccess va, Expr size, int delta, Expr s |
+    size = alloc.getSizeExpr() and
+    s = getASizeCandidate(size) and
+    // Get the unique variable in a size expression like `x` in `malloc(x + 1)`.
+    va = unique( | | getAVariableAccess(s)) and
+    // Compute `delta` as the constant difference between `x` and `x + 1`.
+    bounded1(any(Instruction instr | instr.getUnconvertedResultExpression() = s),
+      any(LoadInstruction load | load.getUnconvertedResultExpression() = va), delta) and
+    n.asExpr() = va and
+    state = delta
+  )
+}
+
+/** Provides the input specification of the `SizeBarrier` module. */
+signature module SizeBarrierInputSig {
+  /** Gets the virtual dispatch branching limit when calculating field flow. */
+  int fieldFlowBranchLimit();
+
+  /** Holds if `source` is a relevant data flow source. */
+  predicate isSource(DataFlow::Node source);
+}
+
+/**
+ * A module that encapsulates a barrier guard to remove false positives from flow like:
+ * ```cpp
+ * char *p = new char[size];
+ * // ...
+ * unsigned n = size;
+ * // ...
+ * if(n < size) {
+ *   use(*p[n]);
+ * }
+ * ```
+ * In this case, the sink pair identified by the product flow library (without any additional barriers)
+ * would be `(p, n)` (where `n` is the `n` in `p[n]`), because there exists a pointer-arithmetic
+ * instruction `pai = a + b` such that:
+ * 1. the allocation flows to `a`, and
+ * 2. `b <= n` where `n` is the `n` in `p[n]`
+ * but because there's a strict comparison that compares `n` against the size of the allocation this
+ * snippet is fine.
+ */
+module SizeBarrier<SizeBarrierInputSig Input> {
+  private module SizeBarrierConfig implements DataFlow::ConfigSig {
+    predicate isSource = Input::isSource/1;
+
+    predicate fieldFlowBranchLimit = Input::fieldFlowBranchLimit/0;
+
+    /**
+     * Holds if `small <= large + k` holds if `g` evaluates to `testIsTrue`.
+     */
+    additional predicate isSink(
+      DataFlow::Node small, DataFlow::Node large, IRGuardCondition g, int k, boolean testIsTrue
+    ) {
+      // The sink is any "large" side of a relational comparison. i.e., the `large` expression
+      // in a guard such as `small <= large + k`.
+      g.comparesLt(small.asOperand(), large.asOperand(), k + 1, true, testIsTrue)
+    }
+
+    predicate isSink(DataFlow::Node sink) { isSink(_, sink, _, _, _) }
+  }
+
+  private module SizeBarrierFlow = DataFlow::Global<SizeBarrierConfig>;
+
+  private int getASizeAddend(DataFlow::Node node) {
+    exists(DataFlow::Node source |
+      SizeBarrierFlow::flow(source, node) and
+      hasSize(_, source, result)
+    )
+  }
+
+  /**
+   * Holds if `small <= large + k` holds if `g` evaluates to `edge`.
+   */
+  private predicate operandGuardChecks(
+    IRGuardCondition g, Operand small, DataFlow::Node large, int k, boolean edge
+  ) {
+    SizeBarrierFlow::flowTo(large) and
+    SizeBarrierConfig::isSink(DataFlow::operandNode(small), large, g, k, edge)
+  }
+
+  /**
+   * Gets an instruction `instr` that is guarded by a check such as `instr <= small + delta` where
+   * `small <= _ + k` and `small` is the "small side" of a relational comparison that checks
+   * whether `small <= size` where `size` is the size of an allocation.
+   */
+  private Instruction getABarrierInstruction0(int delta, int k) {
+    exists(
+      IRGuardCondition g, ValueNumber value, Operand small, boolean edge, DataFlow::Node large
+    |
+      // We know:
+      // 1. result <= value + delta (by `bounded`)
+      // 2. value <= large + k (by `operandGuardChecks`).
+      // So:
+      // result <= value + delta (by 1.)
+      //        <= large + k + delta (by 2.)
+      small = value.getAUse() and
+      operandGuardChecks(pragma[only_bind_into](g), pragma[only_bind_into](small), large,
+        pragma[only_bind_into](k), pragma[only_bind_into](edge)) and
+      bounded(result, value.getAnInstruction(), delta) and
+      g.controls(result.getBlock(), edge) and
+      k < getASizeAddend(large)
+    )
+  }
+
+  /**
+   * Gets an instruction that is guarded by a guard condition which ensures that
+   * the value of the instruction is upper-bounded by size of some allocation.
+   */
+  bindingset[state]
+  pragma[inline_late]
+  Instruction getABarrierInstruction(int state) {
+    exists(int delta, int k |
+      state > k + delta and
+      // result <= "size of allocation" + delta + k
+      //        < "size of allocation" + state
+      result = getABarrierInstruction0(delta, k)
+    )
+  }
+
+  /**
+   * Gets a `DataFlow::Node` that is guarded by a guard condition which ensures that
+   * the value of the node is upper-bounded by size of some allocation.
+   */
+  DataFlow::Node getABarrierNode(int state) {
+    exists(DataFlow::Node source, int delta, int k |
+      SizeBarrierFlow::flow(source, result) and
+      hasSize(_, source, state) and
+      result.asInstruction() = getABarrierInstruction0(delta, k) and
+      state > k + delta
+      // so now we have:
+      // result <= "size of allocation" + delta + k
+      //        < "size of allocation" + state
+    )
+  }
+}