C++: Add a new query for detecting calls to 'c_str' on temporary objects.

Author: MathiasVP

cpp/ql/src/Security/CWE/CWE-416/UseOfStringAfterLifetimeEnds.ql

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+/**
+ * @name Use of string after lifetime ends
+ * @description If the value of a call to 'c_str' outlives the underlying object it may lead to unexpected behavior.
+ * @kind problem
+ * @precision high
+ * @id cpp/use-of-string-after-lifetime-ends
+ * @problem.severity warning
+ * @security-severity 8.8
+ * @tags reliability
+ *       security
+ *       external/cwe/cwe-416
+ *       external/cwe/cwe-664
+ */
+
+import cpp
+import semmle.code.cpp.models.implementations.StdString
+import semmle.code.cpp.models.implementations.StdContainer
+
+/**
+ * Holds if `e` will be consumed by its parent as a glvalue and does not have
+ * an lvalue-to-rvalue conversion. This means that it will be materialized into
+ * a temporary object.
+ */
+predicate isTemporary(Expr e) {
+  e.isPRValueCategory() and
+  e.getUnspecifiedType() instanceof Class and
+  not e.hasLValueToRValueConversion()
+}
+
+/** Holds if `e` is written to a container. */
+predicate isStoredInContainer(Expr e) {
+  exists(StdSequenceContainerInsert insert, Call call, int index |
+    call = insert.getACallToThisFunction() and
+    index = insert.getAValueTypeParameterIndex() and
+    call.getArgument(index) = e
+  )
+  or
+  exists(StdSequenceContainerPush push, Call call, int index |
+    call = push.getACallToThisFunction() and
+    index = push.getAValueTypeParameterIndex() and
+    call.getArgument(index) = e
+  )
+  or
+  exists(StdSequenceEmplace emplace, Call call, int index |
+    call = emplace.getACallToThisFunction() and
+    index = emplace.getAValueTypeParameterIndex() and
+    call.getArgument(index) = e
+  )
+  or
+  exists(StdSequenceEmplaceBack emplaceBack, Call call, int index |
+    call = emplaceBack.getACallToThisFunction() and
+    index = emplaceBack.getAValueTypeParameterIndex() and
+    call.getArgument(index) = e
+  )
+}
+
+/**
+ * Holds if the value of `e` outlives the enclosing full expression. For
+ * example, because the value is stored in a local variable.
+ */
+predicate outlivesFullExpr(Expr e) {
+  any(Assignment assign).getRValue() = e
+  or
+  any(Variable v).getInitializer().getExpr() = e
+  or
+  any(ReturnStmt ret).getExpr() = e
+  or
+  exists(ConditionalExpr cond |
+    outlivesFullExpr(cond) and
+    [cond.getThen(), cond.getElse()] = e
+  )
+  or
+  exists(BinaryOperation bin |
+    outlivesFullExpr(bin) and
+    bin.getAnOperand() = e
+  )
+  or
+  exists(ClassAggregateLiteral aggr |
+    outlivesFullExpr(aggr) and
+    aggr.getAFieldExpr(_) = e
+  )
+  or
+  exists(ArrayAggregateLiteral aggr |
+    outlivesFullExpr(aggr) and
+    aggr.getAnElementExpr(_) = e
+  )
+  or
+  isStoredInContainer(e)
+}
+
+from Call c
+where
+  outlivesFullExpr(c) and
+  not c.isFromUninstantiatedTemplate(_) and
+  c.getTarget() instanceof StdStringCStr and
+  isTemporary(c.getQualifier().getFullyConverted())
+select c,
+  "The underlying string object is destroyed after the call to '" + c.getTarget() + "' returns."

cpp/ql/test/query-tests/Security/CWE/CWE-416/semmle/tests/UseOfStringAfterLifetimeEnds.qlref

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+