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

[Jann Horn]

On Thu, Nov 28, 2019 at 8:18 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Thu, Nov 28, 2019 at 11:07 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > 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.
>
> Actually, that'd still be wrong even with the existing kernel code for
> two reasons:
>
>  - if you reparent to a subreaper, the existing exec_id comparison breaks

... actually, I was wrong about this, this case is fine because the
->exit_signal is reset in reparent_leader().