drm/vmwgfx: Fix double free in vmw_recv_msg()

commit 08b0c891 upstream.

We recently added a kfree() after the end of the loop:

	if (retries == RETRIES) {
		kfree(reply);
		return -EINVAL;
	}

There are two problems.  First the test is wrong and because retries
equals RETRIES if we succeed on the last iteration through the loop.
Second if we fail on the last iteration through the loop then the kfree
is a double free.

When you're reading this code, please note the break statement at the
end of the while loop.  This patch changes the loop so that if it's not
successful then "reply" is NULL and we can test for that afterward.

Cc: <stable@vger.kernel.org>
Fixes: 6b7c3b86 ("drm/vmwgfx: fix memory leak when too many retries have occurred")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/vmwgfx/vmwgfx_msg.c

@@ -353,7 +353,7 @@ static int vmw_recv_msg(struct rpc_channel *channel, void **msg,
 				     !!(HIGH_WORD(ecx) & MESSAGE_STATUS_HB));
 		if ((HIGH_WORD(ebx) & MESSAGE_STATUS_SUCCESS) == 0) {
 			kfree(reply);
-
+			reply = NULL;
 			if ((HIGH_WORD(ebx) & MESSAGE_STATUS_CPT) != 0) {
 				/* A checkpoint occurred. Retry. */
 				continue;
@@ -377,7 +377,7 @@ static int vmw_recv_msg(struct rpc_channel *channel, void **msg,
 
 		if ((HIGH_WORD(ecx) & MESSAGE_STATUS_SUCCESS) == 0) {
 			kfree(reply);
-
+			reply = NULL;
 			if ((HIGH_WORD(ecx) & MESSAGE_STATUS_CPT) != 0) {
 				/* A checkpoint occurred. Retry. */
 				continue;
@@ -389,10 +389,8 @@ static int vmw_recv_msg(struct rpc_channel *channel, void **msg,
 		break;
 	}
 
-	if (retries == RETRIES) {
-		kfree(reply);
+	if (!reply)
 		return -EINVAL;
-	}
 
 	*msg_len = reply_len;
 	*msg     = reply;