From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752655AbdCERpJ (ORCPT ); Sun, 5 Mar 2017 12:45:09 -0500 Received: from mail-ua0-f176.google.com ([209.85.217.176]:34739 "EHLO mail-ua0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751905AbdCERpH (ORCPT ); Sun, 5 Mar 2017 12:45:07 -0500 MIME-Version: 1.0 In-Reply-To: References: <20170304193910.GG29622@ZenIV.linux.org.uk> <20170305155707.GI29622@ZenIV.linux.org.uk> <20170305163301.GJ29622@ZenIV.linux.org.uk> From: Dmitry Vyukov Date: Sun, 5 Mar 2017 18:38:25 +0100 Message-ID: Subject: Re: fs: use-after-free in path_lookupat To: Al Viro Cc: "linux-fsdevel@vger.kernel.org" , LKML , syzkaller Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 5, 2017 at 6:33 PM, Dmitry Vyukov wrote: > On Sun, Mar 5, 2017 at 5:33 PM, Al Viro wrote: >> On Sun, Mar 05, 2017 at 05:14:23PM +0100, Dmitry Vyukov wrote: >>> On Sun, Mar 5, 2017 at 4:57 PM, Al Viro wrote: >>> > On Sun, Mar 05, 2017 at 12:37:13PM +0100, Dmitry Vyukov wrote: >>> > >>> >> I am pretty sure it is that one. >>> >> I don't think I ever used name_to_handle_at syscall in my life and I >>> >> definitely didn't make it lookup a memfd :) >>> > >>> > So what does it normally return? On the runs where we do not hit that >>> > use-after-free, that is. >>> > >>> > What gets triggered there is nd->path.dentry pointing to already freed >>> > dentry. We are in RCU mode, so we are not pinning the dentry and it >>> > might have reached dentry_free(). However, anything with DCACHE_RCUACCESS >>> > set would have freeing RCU-delayed, making that impossible. >>> > >>> > memfd stuff does *not* have DCACHE_RCUACCESS, which would've made it >>> > plausible, but... there we really should've been stopped cold by >>> > the d_can_lookup() check - that is done while we are still holding >>> > a reference to struct file, which should've prevented freeing and >>> > reuse. So at the time of that check we have dentry still not reused >>> > by anything, and d_can_lookup() should've failed. >>> > >>> > There is a race that could bugger the things up in that area, but it needs >>> > empty name, so this one is something else... >>> >>> You can see from the log above that s always empty somehow, so the >>> d_can_lookup check is simply not done. I have not looked at the code, >>> but it's not racy, so should follow from the arguments passed to >>> name_to_handle_at. >> >> Umm... name_to_handle_at() in your log: >> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300", &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0, 0x1000) >> and unless I'm misreading what you are printing there, you have "./bus0" >> passed as the second argument. Right? That's pretty much why I asked about >> other possible calls triggering it... >> >> If you are somehow getting there with empty name and if there's another >> thread closing these memfd descriptors, I understand what's going on there. >> It's how we are getting that empty name on your syscall arguments that >> looks very odd... > > > Added more debug output. > > name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300", > &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0, > 0x1000) > > actually passes name="" because of the overlapping addresses. Flags > contain AT_EMPTY_PATH. The problem can be more general as a bunch of xxxat calls support AT_EMPTY_PATH.