linux-kernel - Re: [PATCH] prctl: add PR_[GS]ET

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Date:   Wed, 01 Aug 2018 09:43:57 +0200
From:   Jürg Billeter <j@...ron.ch>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] prctl: add PR_[GS]ET_KILLABLE

On Tue, 2018-07-31 at 18:26 +0200, Jann Horn wrote:
> On Mon, Jul 30, 2018 at 10:01 AM Jürg Billeter <j@...ron.ch> wrote:
> 
> [...]
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 38509dc1f77b..264de630d548 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> 
> [...]
> > +       case PR_SET_KILLABLE:
> > +               if (arg2 != 1 || arg3 || arg4 || arg5)
> > +                       return -EINVAL;
> > +               me->signal->flags &= ~SIGNAL_UNKILLABLE;
> > +               break;
> 
> I don't have an opinion on this patchset otherwise, but should this
> prctl maybe block PR_SET_KILLABLE if you're actually the real init
> process? This seems like it could potentially lead to weird things.

While I don't expect global init to use this, I can't think of a good
reason to disallow it in the kernel. Do you have specific concerns or
is the code in kernel/fork.c the only reason? I prefer avoiding special
cases unless really required.

> This code in kernel/fork.c seems to rely on the fact that global init
> is SIGNAL_UNKILLABLE, and probably also leads to weirdness if
> container init is non-SIGNAL_UNKILLABLE:

Yes, Oleg has mentioned this as well. I have to change copy_process()
to directly check for the PID namespace root process instead of
checking for SIGNAL_UNKILLABLE.

Jürg