From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C648DC433E0 for ; Thu, 11 Mar 2021 11:46:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78E6B64E45 for ; Thu, 11 Mar 2021 11:46:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232760AbhCKLqP (ORCPT ); Thu, 11 Mar 2021 06:46:15 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52494 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232754AbhCKLqM (ORCPT ); Thu, 11 Mar 2021 06:46:12 -0500 Received: from hr2.samba.org (hr2.samba.org [IPv6:2a01:4f8:192:486::2:0]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AB682C061574 for ; Thu, 11 Mar 2021 03:46:11 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=samba.org; s=42; h=Date:Message-ID:From:To:CC; bh=hfntgB+ERpSRhEccJpUTjpbIyouYPN4eJrG/K8X/tv8=; b=DpYWvFN/obOVglgkQfxq4pc5gu UmWDwLgFaToGhGEpgNRYSPd8RfTCY93L28/Jjds4oBHHfJlWc8UIcWCcyfhUpwYTFqDggInpv0oGX SBk/LoLVBunrrqe0DEpUwTqMoBjzw1RwpZ0KRsfDPtAx5op//uYMUoterzLLNixV9jsz32HEU/eEH Wf65k98Rd3GNpSbC1u/CB0wnbgiNXSbdk0uYNyQFffSo7+9kLbeTLy6td0bAc0Vn71aRm+FQZfvfs cJyzEG2fUjiDvNjrh3EvEaYCcQmxzZ1FmAjDCI7CGxMoyIExTqE7S2R7U0h/v+xPYuRn5arnivgjP jneKaybEp9hzwzgRtXNPU9hxmaZUJwvve9HYskeYMAKOO0j8Azlnro5fSlIKqSV5Z6LiZlaQzby5i V7Q9TVkGiPN1jwGgtMg3FbZTXtl1IwLswoQECtPB3cJzYsrrFKaVqHkaOT1/+5u7Ezi+9CTJRw26a 9ebBsS2DO9ipePGV62fwAf75; Received: from [127.0.0.2] (localhost [127.0.0.1]) by hr2.samba.org with esmtpsa (TLS1.3:ECDHE_RSA_CHACHA20_POLY1305:256) (Exim) id 1lKJlZ-0002Kv-GS; Thu, 11 Mar 2021 11:46:09 +0000 To: Pavel Begunkov , Jens Axboe , io-uring@vger.kernel.org References: From: Stefan Metzmacher Subject: IORING_SETUP_ATTACH_WQ (was Re: [PATCH 1/3] io_uring: fix invalid ctx->sq_thread_idle) Message-ID: <5efea46e-8dce-3d6b-99e4-9ee9a111d8a6@samba.org> Date: Thu, 11 Mar 2021 12:46:09 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Am 11.03.21 um 12:18 schrieb Pavel Begunkov: > On 10/03/2021 13:56, Stefan Metzmacher wrote: >> >> Hi Pavel, >> >> I wondered about the exact same change this morning, while researching >> the IORING_SETUP_ATTACH_WQ behavior :-) >> >> It still seems to me that IORING_SETUP_ATTACH_WQ changed over time. >> As you introduced that flag, can you summaries it's behavior (and changes) >> over time (over the releases). > > Not sure I remember the story in details, but from the beginning it was > for io-wq sharing only, then it had expanded to SQPOLL as well. Now it's > only about SQPOLL sharing, because of the recent io-wq changes that made > it per-task and shared by default. > > In all cases it should be checking the passed in file, that should retain > the old behaviour of failing setup if the flag is set but wq_fd is not valid. Thanks, that's what I also found so far, see below for more findings. >> >> I'm wondering if ctx->sq_creds is really the only thing we need to take care of. > > io-wq is not affected by IORING_SETUP_ATTACH_WQ. It's per-task and mimics > all the resources of the creator (on the moment of io-wq creation). Off > ATTACH_WQ topic, but that's almost matches what it has been before, and > with dropped unshare bit, should be totally same. > > Regarding SQPOLL, it was always using resources of the first task, so > those are just reaped of from it, and not only some particular like > mm/files but all of them, like fork does, so should be safer. > > Creds are just a special case because of that personality stuff, at least > if we add back iowq unshare handling. > >> >> Do we know about existing users of IORING_SETUP_ATTACH_WQ and their use case? > > Have no clue. > >> As mm, files and other things may differ now between sqe producer and the sq_thread. > > It was always using mm/files of the ctx creator's task, aka ctx->sqo_task, > but right, for the sharing case those may be different b/w ctx, so looks > like a regression to me Good. I'll try to explore a possible way out below. Ok, I'm continuing the thread here (just pasting the mail I already started to write :-) I did some more research regarding IORING_SETUP_ATTACH_WQ in 5.12. The current logic in io_sq_offload_create() is this: + /* Retain compatibility with failing for an invalid attach attempt */ + if ((ctx->flags & (IORING_SETUP_ATTACH_WQ | IORING_SETUP_SQPOLL)) == + IORING_SETUP_ATTACH_WQ) { + struct fd f; + + f = fdget(p->wq_fd); + if (!f.file) + return -ENXIO; + if (f.file->f_op != &io_uring_fops) { + fdput(f); + return -EINVAL; + } + fdput(f); + } That means that IORING_SETUP_ATTACH_WQ (without IORING_SETUP_SQPOLL) is completely ignored (except that we still simulate the -ENXIO and -EINVAL cases), correct? (You already agreed on that above :-) The reason for this is that io_wq is no longer maintained per io_ring_ctx, but instead it is now global per io_uring_task. Which means each userspace thread (or the sq_thread) has its own io_uring_task and thus its own io_wq. Regarding the IORING_SETUP_SQPOLL|IORING_SETUP_ATTACH_WQ case we still allow attaching to the sq_thread of a different io_ring_ctx. The sq_thread runs in the context of the io_uring_setup() syscall that created it. We used to switch current->mm, current->files and other things before calling __io_sq_thread() before, but we no longer do that. And this seems to be security problem to me, as it's now possible for the attached io_ring_ctx to start sqe's copying the whole address space of the donator into a registered fixed file of the attached process. As we already ignore IORING_SETUP_ATTACH_WQ without IORING_SETUP_SQPOLL, what about ignoring it as well if the attaching task uses different ->mm, ->files, ... So IORING_SETUP_ATTACH_WQ would only have any effect if the task calling io_uring_setup() runs in the same context (except of the creds) as the existing sq_thread, which means it would work if multiple userspace threads of the same userspace process want to share the sq_thread and its io_wq. Everything else would be stupid (similar to the unshare() cases). But as this has worked before, we just silently ignore IORING_SETUP_ATTACH_WQ is we find a context mismatch and let io_uring_setup() silently create a new sq_thread. What do you think? metze