fix io_destroy()/aio_complete() race

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@kernel.org
Fixes: 0460fef2a921 "aio: use cancellation list lazily"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/aio.c b/fs/aio.c
index 8061d9787e547d305ea6953a092d61df933b332a..49f53516eef090fbdff6c6ae6134a49ac0964daa 100644 (file)
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -634,9 +634,8 @@ static void free_ioctx_users(struct percpu_ref *ref)
        while (!list_empty(&ctx->active_reqs)) {
                req = list_first_entry(&ctx->active_reqs,
                                       struct aio_kiocb, ki_list);
-
-               list_del_init(&req->ki_list);
                kiocb_cancel(req);
+               list_del_init(&req->ki_list);
        }
 
        spin_unlock_irq(&ctx->ctx_lock);