linux-kernel - Re: timerfd: use-after-free in timerfd_remove

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Message-ID: <CACT4Y+b=3q3pf=zEb0t4vKFM2_05SoGZ6r9+08HknTkZBWqtag@mail.gmail.com>
Date:   Tue, 31 Jan 2017 13:09:50 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: timerfd: use-after-free in timerfd_remove_cancel

On Tue, Jan 31, 2017 at 12:45 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 31 Jan 2017, Thomas Gleixner wrote:
>
>> On Mon, 30 Jan 2017, Dmitry Vyukov wrote:
>> >
>> > Seems that ctx->might_cancel is racy.
>>
>> Yes, it is. Fix below.
>
> And the fix is racy as well. Darn, we really need to lock the context to
> avoid that mess.

Yes. I think we need to lock most of timerfd_settime. Otherwise we can
end up with a timer that needs to be in the cancel list, but it is
actually not; or vice versa.