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 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5373EC4345F for ; Sun, 5 May 2024 19:46:18 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 2E9C5112D9A; Sun, 5 May 2024 19:46:17 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=linux.org.uk header.i=@linux.org.uk header.b="WRqMcU/Z"; dkim-atps=neutral Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) by gabe.freedesktop.org (Postfix) with ESMTPS id 9B100112D9A for ; Sun, 5 May 2024 19:46:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=CR8HSnmd0wpNYx92D/iUJFVLON8rnjrI0Q5oSHnNFnM=; b=WRqMcU/ZGHTJPWmlNeLfcAzApr RaTueuuX4UcawZAh6Js5j23MefBc0PwL5TSeboB94b+oL1d96HnFLOApzEMQVEGqnmZxdESFBaNTI gRx9uhQrbDV624FRxrN2uwkjDUiob3b11Q3dQS554W0u4b+UDZ2p6h0mqcGpu0KuODkBE3wJFkYrM Ccn0D4iIrdABSC8UQ6CYmaUJHFbMEVzW2eI8nZeDLQIvgBEdtrYLsDAd1QiS6AKOeAbO3cA3jKEtE HFshm2HNHREdy6EkxPFF0mTgymJ16FhUnBVz1pVnHYdV6myF9x5Uw750zSbEqR1U+RqadTs4p7amr uWFRmDTQ==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1s3hoB-00DGHE-1B; Sun, 05 May 2024 19:46:03 +0000 Date: Sun, 5 May 2024 20:46:03 +0100 From: Al Viro To: Linus Torvalds Cc: Christian Brauner , keescook@chromium.org, axboe@kernel.dk, christian.koenig@amd.com, dri-devel@lists.freedesktop.org, io-uring@vger.kernel.org, jack@suse.cz, laura@labbott.name, linaro-mm-sig@lists.linaro.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, minhquangbui99@gmail.com, sumit.semwal@linaro.org, syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes Message-ID: <20240505194603.GH2118490@ZenIV> References: <202405031110.6F47982593@keescook> <20240503211129.679762-2-torvalds@linux-foundation.org> <20240503212428.GY2118490@ZenIV> <20240504-wohngebiet-restwert-6c3c94fddbdd@brauner> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Sat, May 04, 2024 at 08:53:47AM -0700, Linus Torvalds wrote: > poll_wait > -> __pollwait > -> get_file (*boom*) > > but the boom is very small because the poll_wait() will be undone by > poll_freewait(), and normally poll/select has held the file count > elevated. Not quite. It's not that poll_wait() calls __pollwait(); it calls whatever callback that caller of ->poll() has set for it. __pollwait users (select(2) and poll(2), currently) must (and do) make sure that refcount is elevated; others (and epoll is not the only one) need to do whatever's right for their callbacks. I've no problem with having epoll grab a reference, but if we make that a universal requirement ->poll() instances can rely upon, we'd better verify that *all* vfs_poll() are OK. And that ought to go into Documentation/filesystems/porting.rst ("callers of vfs_poll() must make sure that file is pinned; ->poll() instances may rely upon that, but they'd better be very careful about grabbing extra references themselves - it's acceptable for files on internal mounts, but *NOT* for anything on mountable filesystems. Any instance that does it needs an explicit comment telling not to reuse that blindly." or something along those lines). Excluding epoll, select/poll and callers that have just done fdget() and will do fdput() after vfs_poll(), we have this: drivers/vhost/vhost.c:213: mask = vfs_poll(file, &poll->table); vhost_poll_start(). Might get interesting... Calls working with vq->kick as file seem to rely upon vq->mutex, but I'll need to refresh my memories of that code to check if that's all we need - and then there's vhost_net_enable_vq(), which also needs an audit. fs/aio.c:1738: mask = vfs_poll(req->file, &pt) & req->events; fs/aio.c:1932: mask = vfs_poll(req->file, &apt.pt) & req->events; aio_poll() and aio_poll_wake() resp. req->file here is actually ->ki_filp of iocb that contains work as iocb->req.work; it get dropped only in iocb_destroy(), which also frees iocb. Any call that might've run into req->file not pinned is already in UAF land. io_uring/poll.c:303: req->cqe.res = vfs_poll(req->file, &pt) & req->apoll_events; io_uring/poll.c:622: mask = vfs_poll(req->file, &ipt->pt) & poll->events; Should have req->file pinned, but I'll need to RTFS a bit for details. That, or ask Jens to confirm... net/9p/trans_fd.c:236: ret = vfs_poll(ts->rd, pt); net/9p/trans_fd.c:238: ret = (ret & ~EPOLLOUT) | (vfs_poll(ts->wr, pt) & ~EPOLLIN); p9_fd_poll(); ->rd and ->wr are pinned and won't get dropped until p9_fd_close(), which frees ts immediately afterwards. IOW, if we risk being called with ->rd or ->wr not pinned, we are in UAF land already. Incidentally, what the hell is this in p9_fd_open()? * It's technically possible for userspace or concurrent mounts to * modify this flag concurrently, which will likely result in a * broken filesystem. However, just having bad flags here should * not crash the kernel or cause any other sort of bug, so mark this * particular data race as intentional so that tooling (like KCSAN) * can allow it and detect further problems. */ Why not simply fix the race instead? As in spin_lock(&ts->rd->f_lock); ts->rd->f_flags |= O_NONBLOCK; spin_unlock(&ts->rd->f_lock); and similar for ts->wr? Sigh...