[PATCH 3.18 07/21] fix io_destroy()/aio_complete() race

From: Greg Kroah-Hartman
Date: Tue Jun 12 2018 - 13:45:17 EST

3.18-stable 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).

Cc: stable@xxxxxxxxxx
Fixes: 0460fef2a921 "aio: use cancellation list lazily"
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/aio.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -560,9 +560,8 @@ 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);
+ list_del_init(&req->ki_list);