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Subject: Re: [PATCH RFC] signalfd: add support for SFD_TASK
To:     Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>
Cc:     io-uring <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
References: <254505c9-2b76-ebeb-306c-02aaf1704b88@kernel.dk>
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 <CAG48ez2VBS4bVJqdCU9cUhYePYCiUURvXZWneBx2KGkg3L9d4g@mail.gmail.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <f4144a96-58ef-fba7-79f0-e5178147b6bb@rasmusvillemoes.dk>
Date:   Thu, 28 Nov 2019 10:02:44 +0100
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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?

Rasmus