rbd: Reset positive result codes to zero in object map update path

commit a6c4250 upstream.

In a reply message to an RBD request, a positive result code indicates
a data payload, which is not allowed for writes. While
rbd_osd_req_callback() already resets a positive result code for writes
to zero, rbd_object_map_callback() does not. This allows a corrupted
reply to an object map update to trigger the rbd_assert(*result < 0) in
__rbd_obj_handle_request(). This happens, because
rbd_object_map_callback() calls rbd_obj_handle_request() ->
__rbd_obj_handle_request() and passes this positive result code. From
__rbd_obj_handle_request(), rbd_obj_advance_write() is called, which
leaves the positive result code unchanged and returns true. Therefore,
the if(done && *result) branch is executed in __rbd_obj_handle_request()
and the assertion triggers.

This patch fixes the issue by adjusting the logic in the
rbd_object_map_callback() path. A positive result code for an object map
update is now reset to zero (similar to rbd_osd_req_callback()), and the
message is subsequently handled the same way as if the result code was
zero from the beginning. Additionally, a WARN_ON_ONCE() is added for
this case.

Cc: stable@vger.kernel.org
Fixes: 22e8bd5 ("rbd: support for object-map and fast-diff")
Signed-off-by: Raphael Zimmer <raphael.zimmer@tu-ilmenau.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Raphael Zimmer

drivers/block/rbd.c

@@ -1957,9 +1957,14 @@ static int rbd_object_map_update_finish(struct rbd_obj_request *obj_req,
 	bool has_current_state;
 	void *p;
 
-	if (osd_req->r_result)
+	if (osd_req->r_result < 0)
 		return osd_req->r_result;
 
+	/*
+	 * Writes aren't allowed to return a data payload.
+	 */
+	WARN_ON_ONCE(osd_req->r_result > 0);
+
 	/*
 	 * Nothing to do for a snapshot object map.
 	 */