linux-kernel - Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.

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Message-ID: <CAGXu5j+F3zUZ6+8cKuurq2fV8L5Piy5dvYrYXeFVG0kVmpZMRw@mail.gmail.com>
Date:   Mon, 9 Jul 2018 11:16:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-doc@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.

On Fri, Jul 6, 2018 at 8:29 PM, NeilBrown <neilb@...e.com> wrote:
>
> Previous patch ("VFS: simplify seq_file iteration code and interface")
> removed code to set ->from to zero when ->count is zero, as ->from is
> dead at that time.  However it didn't ensure ->from was set properly
> whenever ->count becomes non-zero.
> This can only happen when ->show() is called.  Of the three places it
> is called one already has ->from set to zero.  The other two are
> fixed by setting from to zero after fully flushing the buffer (at which
> point ->count will also be zero).
>
> Reported-by: Jann Horn <jannh@...gle.com>
> Signed-off-by: NeilBrown <neilb@...e.com>

I *think* this solves this report, which looks very much like Jann's reproducer:

https://syzkaller.appspot.com/bug?extid=4b712dce5cbce6700f27

-Kees

-- 
Kees Cook
Pixel Security