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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 CA642C6778A for ; Tue, 24 Jul 2018 05:17:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 80B0C20852 for ; Tue, 24 Jul 2018 05:17:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 80B0C20852 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ZenIV.linux.org.uk Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388178AbeGXGWJ (ORCPT ); Tue, 24 Jul 2018 02:22:09 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:39052 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726098AbeGXGWJ (ORCPT ); Tue, 24 Jul 2018 02:22:09 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.87 #1 (Red Hat Linux)) id 1fhphO-0000ao-GU; Tue, 24 Jul 2018 05:17:26 +0000 Date: Tue, 24 Jul 2018 06:17:26 +0100 From: Al Viro To: "Dae R. Jeong" Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, byoungyoung@purdue.edu, kt0755@gmail.com, bammanag@purdue.edu Subject: Re: KASAN: use-after-free Read in link_path_walk Message-ID: <20180724051726.GH30522@ZenIV.linux.org.uk> References: <20180724034542.GA19283@dragonet> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180724034542.GA19283@dragonet> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 24, 2018 at 12:45:42PM +0900, Dae R. Jeong wrote: > Diagnosis: > We think that it is possible that link_path_walk() dereferences a > freed pointer when cleanup_mnt() is executed between path_init() and > link_path_walk(). > > Since I'm not an expert on a file system and don't fully understand > the crash, please see a executed program and a crash log below in > case that my understanding is wrong. > > > Executed Program: > Thread0 Thread1 > mkdir("./file0") > |--------------------------| > | mount("./file0", "./file0", "devpts", 0x0, "") > | | > openat(AT_FDCWD, chroot("./file0") > "/dev/vcs", 0x200, 0x0) umount("./file0", 0x2) > > openat(), chroot(), umount() syscalls are executed after mount() syscall. > We think a race occurs between openat() and chroot() because RaceFuzzer > executed openat() and chroot() concurrently. > > > (Possible) Thread interleaving: > CPU0 (path_openat) CPU1 (cleanup_mnt) > ===== ===== > s = path_init(nd, flags); > if (IS_ERR(s)) { > put_filp(file); > return ERR_CAST(s); > } > > deactivate_super(mnt->mnt.mnt_sb); > > while (!(error = link_path_walk(s, nd)) && > > // (in link_path_walk()) > struct dentry *parent = nd->path.dentry; > nd->flags &= ~LOOKUP_JUMPED; > if (unlikely(parent->d_flags & DCACHE_OP_HASH)) { // UAF occured Do we have LOOKUP_RCU in nd->flags at that point? And how in hell did we get that dentry there? In LOOKUP_RCU mode no freeing should be happening until after we call rcu_read_unlock(), unless the final dput() has happened before rcu_read_lock(). In which case we shouldn't have gotten to that dentry in the first place. And in non-LOOKUP_RCU mode we are bloody well holding references to everything (vfsmount and dentry alike), so that deactivate_super() shouldn't have been called as long as we are holding that reference. Details, please. Ideally - how to reproduce that.