From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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 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> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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.