linux-kernel - Re: [PATCH RFC] signalfd: add support for SFD

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Message-ID: <CAG48ez1v5EmuSvn+LY8od_ZMt1QVdUWqi9DWLSp0CgMxkL=sNg@mail.gmail.com>
Date:   Thu, 28 Nov 2019 11:07:46 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Jens Axboe <axboe@...nel.dk>, io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH RFC] signalfd: add support for SFD_TASK

On Thu, Nov 28, 2019 at 10:02 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> On 28/11/2019 00.27, Jann Horn wrote:
>
> > One more thing, though: We'll have to figure out some way to
> > invalidate the fd when the target goes through execve(), in particular
> > if it's a setuid execution. Otherwise we'll be able to just steal
> > signals that were intended for the other task, that's probably not
> > good.
> >
> > So we should:
> >  a) prevent using ->wait() on an old signalfd once the task has gone
> > through execve()
> >  b) kick off all existing waiters
> >  c) most importantly, prevent ->read() on an old signalfd once the
> > task has gone through execve()
> >
> > We probably want to avoid using the cred_guard_mutex here, since it is
> > quite broad and has some deadlocking issues; it might make sense to
> > put the update of ->self_exec_id in fs/exec.c under something like the
> > siglock,
>
> What prevents one from exec'ing a trivial helper 2^32-1 times before
> exec'ing into the victim binary?

Uh, yeah... that thing should probably become 64 bits wide, too.