C++: Fix false positives involving STDIN_FILENO.

Author: geoffw0

cpp/ql/src/Security/CWE/CWE-311/CleartextTransmission.ql

@@ -21,6 +21,12 @@ import semmle.code.cpp.valuenumbering.GlobalValueNumbering
  * A function call that sends or receives data over a network.
  */
 abstract class NetworkSendRecv extends FunctionCall {
+  /**
+   * Gets the expression for the socket or similar object used for sending or
+   * receiving data.
+   */
+  abstract Expr getSocketExpr();
+
   /**
    * Gets the expression for the buffer to be sent from / received into.
    */
@@ -38,6 +44,8 @@ class NetworkSend extends NetworkSendRecv {
         .hasGlobalName(["send", "sendto", "sendmsg", "write", "writev", "pwritev", "pwritev2"])
   }
 
+  override Expr getSocketExpr() { result = this.getArgument(0) }
+
   override Expr getDataExpr() { result = this.getArgument(1) }
 }
 
@@ -52,9 +60,15 @@ class NetworkRecv extends NetworkSendRecv {
           ])
   }
 
+  override Expr getSocketExpr() { result = this.getArgument(0) }
+
   override Expr getDataExpr() { result = this.getArgument(1) }
 }
 
 from NetworkSendRecv transmission, SensitiveExpr e
-where DataFlow::localFlow(DataFlow::exprNode(e), DataFlow::exprNode(transmission.getDataExpr()))
+where
+  DataFlow::localFlow(DataFlow::exprNode(e), DataFlow::exprNode(transmission.getDataExpr())) and
+  not exists(Zero zero |
+    DataFlow::localFlow(DataFlow::exprNode(zero), DataFlow::exprNode(transmission.getSocketExpr()))
+  )
 select transmission, e

cpp/ql/test/query-tests/Security/CWE/CWE-311/semmle/tests/CleartextTransmission.expected

@@ -5,4 +5,3 @@
 | test3.cpp:70:3:70:6 | call to send | test3.cpp:68:21:68:29 | password1 |
 | test3.cpp:77:3:77:6 | call to recv | test3.cpp:75:15:75:22 | password |
 | test3.cpp:95:3:95:6 | call to read | test3.cpp:95:12:95:19 | password |
-| test3.cpp:102:3:102:6 | call to read | test3.cpp:102:12:102:19 | password |

cpp/ql/test/query-tests/Security/CWE/CWE-311/semmle/tests/test3.cpp

@@ -99,7 +99,7 @@ void test_read()
 		char password[256];
 		int fd = STDIN_FILENO;
 
-		read(fd, password, 256); // GOOD: `password` is received from stdin, not a network socket [FALSE POSITIVE]
+		read(fd, password, 256); // GOOD: `password` is received from stdin, not a network socket
 	}
 }