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=-3.9 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 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 DFB35C433E7 for ; Fri, 9 Oct 2020 12:35:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8235E222B8 for ; Fri, 9 Oct 2020 12:35:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="WAns9c/z" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387506AbgJIMfk (ORCPT ); Fri, 9 Oct 2020 08:35:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35748 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727737AbgJIMfk (ORCPT ); Fri, 9 Oct 2020 08:35:40 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E1341C0613D2; Fri, 9 Oct 2020 05:35:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=XGJ2pvgX56gVRYnrRVMGR+GD7jxHoOmPhdCN2nGgouw=; b=WAns9c/zAxoqwnw07v+/Bc7FWV Mhe8QGYrFGgcYu9BWNrONbC6WKTjFdTTN8n4/KE4GHh0GIyK44aAWBzsHomMunAQXreb3rrN5vzM8 zh6JiOswUIW21ehAUTNdFSI5RkyZUTreNy1+KIcDT8CQ44FxtqvY5zKzzHDGUvyS8C1QGi4eiWgnT gyr14JnJFloqNmUWjUcTJinAmc2Mv6z7t2NjuNxgeVjl4LqtijqrthE6z59ZBcGTcxWmMpxArrEQt g7P52KMSjKXHymHLtntW0subRaXwTLvmUvKDjql8F7wtAQiKWyP7LWF8NCksoH6AO7PFCCnkNp05/ MohsRPZQ==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQrcX-0007Lz-Pw; Fri, 09 Oct 2020 12:35:37 +0000 Date: Fri, 9 Oct 2020 13:35:37 +0100 From: Matthew Wilcox To: Pavel Begunkov Cc: syzbot , axboe@kernel.dk, io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk Subject: Re: KASAN: use-after-free Read in __io_uring_files_cancel Message-ID: <20201009123537.GR20115@casper.infradead.org> References: <0000000000001a684d05b1385e71@google.com> <3a98a77a-a507-954a-f2ec-e38af18c168f@gmail.com> <20201009121211.GQ20115@casper.infradead.org> <6da35bc9-d072-c18b-2268-15d37fa786df@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6da35bc9-d072-c18b-2268-15d37fa786df@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 09, 2020 at 03:28:54PM +0300, Pavel Begunkov wrote: > On 09/10/2020 15:12, Matthew Wilcox wrote: > >> It seems this fails on "node->shift" in xas_next_entry(), that would > >> mean that the node itself was freed while we're iterating on it. > >> > >> __io_uring_files_cancel() iterates with xas_next_entry() and creates > >> XA_STATE once by hand, but it also removes entries in the loop with > >> io_uring_del_task_file() -> xas_store(&xas, NULL); without updating > >> the iterating XA_STATE. Could it be the problem? See a diff below > > > > No, the problem is that the lock is dropped after calling > > xas_next_entry(), and at any point after calling xas_next_entry(), > > the node that it's pointing to can be freed. > > Only the task itself clears/removes entries, others can only insert. > So, could it be freed even though there are no parallel erases? Not with today's implementation, but that's something that might change in the future. I agree it's probably the task itself that's deleting the entry and causing the node to be deleted.