Re: [PATCH RFC] signalfd: add support for SFD_TASK

[Jann Horn]

On Thu, Nov 28, 2019 at 10:02 AM Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> 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.