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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, 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 4CE21C433EA for ; Fri, 17 Jul 2020 11:45:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27E0720684 for ; Fri, 17 Jul 2020 11:45:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="La4PoFUS" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726843AbgGQLph (ORCPT ); Fri, 17 Jul 2020 07:45:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46970 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726079AbgGQLph (ORCPT ); Fri, 17 Jul 2020 07:45:37 -0400 Received: from mail-qv1-xf41.google.com (mail-qv1-xf41.google.com [IPv6:2607:f8b0:4864:20::f41]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 44939C061755; Fri, 17 Jul 2020 04:45:37 -0700 (PDT) Received: by mail-qv1-xf41.google.com with SMTP id ed14so4095569qvb.2; Fri, 17 Jul 2020 04:45:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:reply-to:mime-version :content-disposition; bh=RGmCnCujTCPHPAqseUf5nHct0c3cR5+Pl0sn8j65GRA=; b=La4PoFUSodowSJLHupAbyoCryYe4IscT0uVjcLdnBdcKu34q4TNYPqzS0Nr/jQc+EM F2XyfYyxL8FQhKctiFagZGB9yxdA12JMq8JEvISZeKuMS1vZ5/Oau3CFZLgR8K43xiUU E86yO3eN5U8q9hW+83UrqolEyLOq60sn5RWpuN0fRb6KFwI9m265IqtAwKVQfl4cygy3 0jKEYBIANHgXGZAdahfISR5yQA6FuB5q/EXPJLjCM6LtRPdNK0IPKb1/gbZHs22PgaEY wX7lul5113brAi0ptNWB69Sc4TE4p22uWbGzG5GisCJcFbMfc7QZcO5iTG1ztdjRVt0Y Uktg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:reply-to :mime-version:content-disposition; bh=RGmCnCujTCPHPAqseUf5nHct0c3cR5+Pl0sn8j65GRA=; b=qGMcyCf5HiazK1TIB0hEeejdA3gHvOawaLPnyms2I80MHbS8qx+o82rYiqdlXZqGh9 uQ5m45QAM8osavfd8fGRENDtx5tn/ZC+BQnWASw5CozGSRLCv3P1BKgxernNSPlQG9t9 yNmzTbUayGQqaOXmynwmmr5iiTyctbQMfboKgsKnnHRvV+lHE+s82oxgRpWQRVtzPLac 3HFO243rZmJKbIS/t+Xwtf4YQuHe4MvutB4I+Z51/IOetbxdqF2n8fs5zAzNBL7cKrf8 yOZ7ZRyScGsqvrb3/NPEtql26mXxDLaTdTfuF+3rHq1g5AJ/xFfK8dMVcQNiN6SA6235 QlEg== X-Gm-Message-State: AOAM531570kqhHP5igGwvvl+K2wnd1utHryrtT/hwfmxrojj0j/iwn7n kCwVuEussoQoreJ4MQ/jPMSPFLuE0k3W X-Google-Smtp-Source: ABdhPJzKXuvJZlL9/a9hkaENPQ8WnD+XhRB2h7wtRwtCk65prLKd7UbjOiRuatykH0KlTfyqjIJ2+Q== X-Received: by 2002:a0c:a992:: with SMTP id a18mr8145355qvb.211.1594986336193; Fri, 17 Jul 2020 04:45:36 -0700 (PDT) Received: from PWN (c-76-119-149-155.hsd1.ma.comcast.net. [76.119.149.155]) by smtp.gmail.com with ESMTPSA id s8sm10967968qtc.17.2020.07.17.04.45.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Jul 2020 04:45:35 -0700 (PDT) Date: Fri, 17 Jul 2020 07:45:32 -0400 From: Peilin Ye To: Daniel Colascione Cc: timmurray@google.com, selinux@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, viro@zeniv.linux.org.uk, paul@paul-moore.com, nnk@google.com, sds@tycho.nsa.gov, lokeshgidra@google.com, jmorris@namei.org Subject: Reporting a use-after-free read bug in userfaultfd_release() Message-ID: <20200717114532.GA688728@PWN> Reply-To: 20200401213903.182112-4-dancol@google.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hi all, Syzbot reported the following use-after-free bug in userfaultfd_release(): https://syzkaller.appspot.com/bug?id=4b9e5aea757b678d9939c364e50212354a3480a6 It seems to be caused by this patch. I took a look at the stack trace. In the patch: fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); if (fd < 0) { fput(file); goto out; } If get_unused_fd_flags() fails, `ctx` is freed. Later however, before returning back to userland, userfaultfd_release() is called and tries to use `ctx` again, causing a use-after-free bug. The syzbot reproducer does a setrlimit() then a userfaultfd(). The former sets a hard limit on number of open files to zero, which causes get_unused_fd_flags() to fail. Thank you, Peilin Ye