[PATCH 3.16 345/366] fix io_destroy()/aio_complete() race

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 4faa99965e027cc057c5145ce45fa772caa04e8d upstream.

If io_destroy() gets to cancelling everything that can be cancelled and
gets to kiocb_cancel() calling the function driver has left in ->ki_cancel,
it becomes vulnerable to a race with IO completion.  At that point req
is already taken off the list and aio_complete() does *NOT* spin until
we (in free_ioctx_users()) releases ->ctx_lock.  As the result, it proceeds
to kiocb_free(), freing req just it gets passed to ->ki_cancel().

Fix is simple - remove from the list after the call of kiocb_cancel().  All
instances of ->ki_cancel() already have to cope with the being called with
iocb still on list - that's what happens in io_cancel(2).

Fixes: 0460fef2a921 "aio: use cancellation list lazily"
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/aio.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -565,9 +565,8 @@ static void free_ioctx_users(struct perc
 	while (!list_empty(&ctx->active_reqs)) {
 		req = list_first_entry(&ctx->active_reqs,
 				       struct kiocb, ki_list);
-
-		list_del_init(&req->ki_list);
 		kiocb_cancel(req);
+		list_del_init(&req->ki_list);
 	}
 
 	spin_unlock_irq(&ctx->ctx_lock);