From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752673AbdCEQPf (ORCPT <rfc822;w@1wt.eu>);
        Sun, 5 Mar 2017 11:15:35 -0500
Received: from mail-ua0-f172.google.com ([209.85.217.172]:36112 "EHLO
        mail-ua0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752290AbdCEQPb (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sun, 5 Mar 2017 11:15:31 -0500
MIME-Version: 1.0
In-Reply-To: <20170305155707.GI29622@ZenIV.linux.org.uk>
References: <CACT4Y+bWe1bhVKmH4v1RrhQYUKzwu9BG1COaffx-4J2nnzPO-A@mail.gmail.com>
 <20170304193910.GG29622@ZenIV.linux.org.uk> <CACT4Y+Y23b8QhB8Hmasr5jXtwAwvvY=wVrorgRHzUWcdspURMg@mail.gmail.com>
 <CACT4Y+YNak+YLMpEGrTP5iwtrKB=Csd25fWKAT9hLYRYY6YzGQ@mail.gmail.com>
 <CACT4Y+ZNiki1Xdpe_m-=Tk+7TpeZNnvV-tq=cNai=Xs7dr0rbw@mail.gmail.com>
 <CACT4Y+ZHgE2r9fGsDS_7CHsxa53yrsx0Zp0TBvH4oeqg24w5Yg@mail.gmail.com> <20170305155707.GI29622@ZenIV.linux.org.uk>
From: Dmitry Vyukov <dvyukov@google.com>
Date: Sun, 5 Mar 2017 17:14:23 +0100
Message-ID: <CACT4Y+b2mfMmm1evdWn7PHkNOe1xhwtxqtUB83oawNRQ1ZzbKA@mail.gmail.com>
Subject: Re: fs: use-after-free in path_lookupat
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sun, Mar 5, 2017 at 4:57 PM, Al Viro <viro@zeniv.linux.org.uk> 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.