Adds another rule for null deref

cpp/ql/src/experimental/Likely Bugs/DerefNullResult.cpp

@@ -0,0 +1,23 @@
+char * create (int arg) {
+    if (arg > 42) {
+        // this function may return NULL
+        return NULL;
+    }
+    char * r = malloc(arg);
+    snprintf(r, arg -1, "Hello");
+    return r;
+}
+
+void process(char *str) {
+    // str is dereferenced
+    if (str[0] == 'H') {
+        printf("Hello H\n");
+    }
+}
+
+void test(int arg) {
+    // first function returns a pointer that may be NULL
+    char *str = create(arg);
+    // str is not checked for nullness before being passed to process function
+    process(str);
+}

cpp/ql/src/experimental/Likely Bugs/DerefNullResult.qhelp

@@ -0,0 +1,26 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>This rule finds a dereference of a function parameter, whose value comes from another function call that may return NULL, without checks in the meantime.</p>
+</overview>
+
+<recommendation>
+<p>A check should be added between the return of the function which may return NULL, and its use by the function dereferencing ths pointer.</p>
+</recommendation>
+
+<example>
+<sample src="DerefNullResult.cpp" />
+</example>
+
+<references>
+<li>
+  <a href="https://www.owasp.org/index.php/Null_Dereference">
+    Null Dereference
+  </a> 
+</li>
+</references>
+
+</qhelp>

cpp/ql/src/experimental/Likely Bugs/DerefNullResult.ql

@@ -0,0 +1,34 @@
+/**
+ * @name Null dereference from a function result
+ * @description A function parameter is dereferenced,
+ *              while it comes from a function that may return NULL,
+ *              and is not checked for nullness by the caller.
+ * @kind problem
+ * @id cpp/deref-null-result
+ * @problem.severity recommendation
+ * @tags reliability
+ *       security
+ *       external/cwe/cwe-476
+ */
+
+import cpp
+import semmle.code.cpp.dataflow.new.DataFlow
+
+from Function nuller, Parameter pd, FunctionCall fc, Variable v
+where
+  mayReturnNull(nuller) and
+  functionDereferences(pd.getFunction(), pd.getIndex()) and
+  // there is a function call which will deref parameter pd
+  fc.getTarget() = pd.getFunction() and
+  // the parameter pd comes from a variable v
+  DataFlow::localFlow(DataFlow::exprNode(v.getAnAccess()),
+    DataFlow::exprNode(fc.getArgument(pd.getIndex()))) and
+  // this variable v was assigned by a call to the nuller function
+  unique( | | v.getAnAssignedValue()) = nuller.getACallToThisFunction() and
+  // this variable v is not accessed for an operation (check for NULLness)
+  not exists(VariableAccess vc |
+    vc.getTarget() = v and
+    (vc.getParent() instanceof Operation or vc.getParent() instanceof IfStmt)
+  )
+select fc, "This function call may deref $@ when it can be NULL from $@", v, v.getName(), nuller,
+  nuller.getName()