On Wed, May 23, 2018 at 01:49:04AM +0100, Al Viro wrote: > > Looks like we want to call ->ki_cancel() *BEFORE* removing from the list, > > as well as doing fput() after aio_complete(). The same ordering, BTW, goes > > for aio_read() et.al. > > > > Look: > > CPU1: io_cancel() grabs ->ctx_lock, finds iocb and removes it from the list. > > CPU2: aio_rw_complete() on that iocb. Since the sucker is not in the list > > anymore, we do NOT spin on ->ctx_lock and proceed to free iocb > > CPU1: pass freed iocb to ->ki_cancel(). BOOM. > > BTW, it seems that the mainline is vulnerable to this one. I might be > missing something, but... It is, but with a different attack vector - io_cancel(2) won't do it (it does not remove from the list at all), but io_destroy(2) bloody well will. IMO, we need this in mainline; unless somebody has a problem with it, to #fixes it goes: 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@xxxxxxxxxx Fixes: 0460fef2a921 "aio: use cancellation list lazily" Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> --- diff --git a/fs/aio.c b/fs/aio.c index 8061d9787e54..49f53516eef0 100644 --- 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); -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html