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

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Date:   Tue, 12 Sep 2017 20:54:55 +0200
From:   Jürg Billeter <j@...ron.ch>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH] prctl: add PR_[GS]ET_PDEATHSIG_PROC

Hi Oleg,

Thanks for the review.

On Tue, 2017-09-12 at 19:05 +0200, Oleg Nesterov wrote:
> On 09/09, Jürg Billeter wrote:
> > Unlike
> > PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole
> > subtree without race conditions.
> 
> but I am still not sure this is right... at least I can't understand the
> "without race conditions" above.
> 
> IOW, the child can do prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) right after fork(),
> why this is not enough to kill a whole subtree without race conditions?

What if the parent dies between fork() and prctl()? Besides avoiding
this race condition, it also makes it relatively easy to enforce
PDEATHSIG_PROC for all descendants of a process. You simply set
PDEATHSIG_PROC and then block further changes using seccomp (and set
no_new_privs) to avoid runaway children.

> OTOH. If you want to kill a whole sub-tree then perhaps the exiting process
> should simply send the ->pdeath_signal_proc to the whole sub-tree? Not that
> I really think this makes more sense, but if we add the new API we should
> discuss everything we can.

While this would likely work for my use case of avoiding runaway
processes, I don't think it would make sense for non-SIGKILL use cases
of cooperating processes. Inheritance across fork still allows
resetting PDEATHSIG_PROC in the child after fork and I don't expect the
parent death race to be a significant issue in the case of cooperating
processes.

> Say, CLONE_PARENT. Should it succeed if ->pdeath_signal_proc != 0 ?

Yes, I don't see an issue with that. The new process will be a sibling
and inheriting pdeath_signal_proc seems sensible to me for this.

Jürg