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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 A56F2C11F67 for ; Wed, 14 Jul 2021 09:14:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8A42E61370 for ; Wed, 14 Jul 2021 09:14:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238337AbhGNJRr (ORCPT ); Wed, 14 Jul 2021 05:17:47 -0400 Received: from mail.kernel.org ([198.145.29.99]:48056 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238147AbhGNJRo (ORCPT ); Wed, 14 Jul 2021 05:17:44 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 2B6D961369; Wed, 14 Jul 2021 09:14:48 +0000 (UTC) Date: Wed, 14 Jul 2021 11:14:46 +0200 From: Christian Brauner To: Linus Torvalds Cc: syzbot , brauner@kernel.org, Dmitry Vyukov , Greg Kroah-Hartman , gscrivan@redhat.com, Christoph Hellwig , linux-fsdevel , Linux Kernel Mailing List , stable-commits@vger.kernel.org, stable , syzkaller-bugs , Al Viro Subject: Re: [syzbot] KASAN: null-ptr-deref Read in filp_close (2) Message-ID: <20210714091446.vt4ieretnkjzi7qg@wittgenstein> References: <00000000000069c40405be6bdad4@google.com> <000000000000b00c1105c6f971b2@google.com> <20210714075925.jtlfrhhuj4bzff3m@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210714075925.jtlfrhhuj4bzff3m@wittgenstein> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 14, 2021 at 09:59:25AM +0200, Christian Brauner wrote: > On Tue, Jul 13, 2021 at 11:49:14AM -0700, Linus Torvalds wrote: > > On Mon, Jul 12, 2021 at 9:12 PM syzbot > > wrote: > > > > > > syzbot has found a reproducer for the following issue on: > > > > Hmm. > > > > This issue is reported to have been already fixed: > > > > Fix commit: 9b5b8722 file: fix close_range() for unshare+cloexec > > > > and that fix is already in the reported HEAD commit: > > > > > HEAD commit: 7fef2edf sd: don't mess with SD_MINORS for CONFIG_DEBUG_BL.. > > > > and the oops report clearly is from that: > > > > > CPU: 1 PID: 8445 Comm: syz-executor493 Not tainted 5.14.0-rc1-syzkaller #0 > > > > so the alleged fix is already there. > > > > So clearly commit 9b5b872215fe ("file: fix close_range() for > > unshare+cloexec") does *NOT* fix the issue. > > > > This was originally bisected to that 582f1fb6b721 ("fs, close_range: > > add flag CLOSE_RANGE_CLOEXEC") in > > > > https://syzkaller.appspot.com/bug?id=1bef50bdd9622a1969608d1090b2b4a588d0c6ac > > > > which is where the "fix" is from. > > > > It would probably be good if sysbot made this kind of "hey, it was > > reported fixed, but it's not" very clear. > > > > The KASAN report looks like a use-after-free, and that "use" is > > actually the sanity check that the file count is non-zero, so it's > > really a "struct file *" that has already been free'd. > > > > That bogus free is a regular close() system call > > > > > filp_close+0x22/0x170 fs/open.c:1306 > > > close_fd+0x5c/0x80 fs/file.c:628 > > > __do_sys_close fs/open.c:1331 [inline] > > > __se_sys_close fs/open.c:1329 [inline] > > > > And it was opened by a "creat()" system call: > > > > > Allocated by task 8445: > > > __alloc_file+0x21/0x280 fs/file_table.c:101 > > > alloc_empty_file+0x6d/0x170 fs/file_table.c:150 > > > path_openat+0xde/0x27f0 fs/namei.c:3493 > > > do_filp_open+0x1aa/0x400 fs/namei.c:3534 > > > do_sys_openat2+0x16d/0x420 fs/open.c:1204 > > > do_sys_open fs/open.c:1220 [inline] > > > __do_sys_creat fs/open.c:1294 [inline] > > > __se_sys_creat fs/open.c:1288 [inline] > > > __x64_sys_creat+0xc9/0x120 fs/open.c:1288 > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > > > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 > > > entry_SYSCALL_64_after_hwframe+0x44/0xae > > > > But it has apparently already been closed from a workqueue: > > > > > Freed by task 8445: > > > __fput+0x288/0x920 fs/file_table.c:280 > > > task_work_run+0xdd/0x1a0 kernel/task_work.c:164 > > > > So it's some kind of confusion and re-use of a struct file pointer. > > > > Which is certainly consistent with the "fix" in 9b5b872215fe ("file: > > fix close_range() for unshare+cloexec"), but it very much looks like > > that fix was incomplete and not the full story. > > > > Some fdtable got re-allocated? The fix that wasn't a fix ends up > > re-checking the maximum file number under the file_lock, but there's > > clearly something else going on too. > > > > Christian? > > Looking into this now. > > I have to say I'm very confused by the syzkaller report here. > > If I go to > > https://syzkaller.appspot.com/bug?extid=283ce5a46486d6acdbaf > > which is the original link in the report it shows me > > android-54 KASAN: use-after-free Read in filp_close C 2 183d 183d 0/1 upstream: reported C repro on 2021/01/11 12:38 > > which seems to indicate that this happened on an Android specific 5.4 > kernel? > > But ok, so I click on the link "upstream: reported C repro on 2021/01/11 12:38" > which takes me to a google group > > https://groups.google.com/g/syzkaller-android-bugs/c/FQj0qcRSy_M/m/wrY70QFzBAAJ > > which again strongly indicates that this is an Android specific kernel? > > HEAD commit: c9951e5d Merge 5.4.88 into android12-5.4 > git tree: android12-5.4 > > but then I can click on the dashboard link for that crash report and it > takes me to: > > https://syzkaller.appspot.com/bug?extid=53897bcb31b82c7a08fe > > which seems to be the upstream report? > > So I'm a bit confused whether I'm even looking at the correct bug report > but I'll just give the repro a try and see what's going on. Ok, reproduced and I think I found the issue. It's not related to close_fd((), I think it's caused by a UAF when FSCONFIG_SET_FD is with the key "source" and a valid fd passed through "aux". Briefly, fs_parameter is a union: struct fs_parameter { const char *key; /* Parameter name */ enum fs_value_type type:8; /* The type of value here */ union { char *string; void *blob; struct filename *name; struct file *file; }; size_t size; int dirfd; }; and cgroup1_parse_param is copying out param->string when the param's key is "source" without verifying that param->type is actually fs_value_is_string. I'll explain in detail in the commit once I've confirmed and tested that my suspicion is correct. Christian