linux-kernel - Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect PIDFD

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Date: Tue, 20 Feb 2024 12:00:12 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tycho Andersen <tycho@...ho.pizza>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect
 PIDFD_THREAD

On 02/20, Christian Brauner wrote:
>
> On Tue, Feb 20, 2024 at 10:02:56AM +0100, Oleg Nesterov wrote:
> >
> > Ah. IIRC criu uses this hack to restore the pending (arbitrary) signals
> > collected at dump time.
> >
> > I was a bit surprise sys_pidfd_send_signal() allows this hack too, I don't
>
> I think that we simply mirrored the restrictions in the other system
> calls.
>
> > think that criu uses pidfd at restore time, but I do not know.
>
> Hm, I just checked and it doesn't use pidfd_send_signal(). It uses
> pidfds but only for pid reuse detection for RPC clients.

But perhaps something else already uses pidfd_send_signal() with info != NULL
or with info->si_code == SI_USER, we can't know. Please see below.

> So right now si_code is blocked for >= 0 and for SI_TKILL. If we were to
> simply ensure that si_code can't be < 0 then this amounts to effectively
> blocking @info from being filled in by userspace at all. Because 0 is a
> valid value.

I'am afraid I misunderstand you again... 0 == SI_USER is not a valid value
when siginfo != NULL.

Perhaps we can kill the "task_pid(current) != pid" check and just return
EPERM if "kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL", I don't think
anobody needs pidfd_send_send_signal() to signal yourself. See below.

> +       /* Currently unused. */
> +       if (info)
> +               return -EINVAL;

Well, to me this looks like the unnecessary restriction... And why?

But whatever we do,

> -               /* Only allow sending arbitrary signals to yourself. */
> -               ret = -EPERM;
> -               if ((task_pid(current) != pid) &&
> -                   (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
> -                       goto err;

Can I suggest to fix this check in your tree (add type > PIDTYPE_TGID as
we discussed) first, then do other changes on top?

This way we can revert the next change(s) if we get regressions reports
without re-introducing the security problem.

Oleg.