fix io_destroy()/aio_complete() race

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).

Change-Id: I666f1c947f37cd11593d71e0711301c0dd1e0cd6
Cc: stable@kernel.org
Fixes: 0460fef2a9 "aio: use cancellation list lazily"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/aio.c

@@ -297,8 +297,8 @@ static void free_ioctx(struct kioctx *ctx)
 		req = list_first_entry(&ctx->active_reqs,
 				       struct kiocb, ki_list);
 
-		list_del_init(&req->ki_list);
 		kiocb_cancel(ctx, req, &res);
+		list_del_init(&req->ki_list);
 	}
 
 	spin_unlock_irq(&ctx->ctx_lock);