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

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Date:   Mon, 09 Oct 2017 11:32:13 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jürg Billeter <j@...ron.ch>
Cc:     Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Filipe Brandenburger <filbranden@...gle.com>,
        David Wilcox <davidvsthegiant@...il.com>, hansecke@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] prctl: add PR_[GS]ET_PDEATHSIG_PROC

Jürg Billeter <j@...ron.ch> writes:

> On Thu, 2017-10-05 at 18:27 +0200, Oleg Nesterov wrote:
>> On 10/03, Jürg Billeter wrote:
>> > 
>> > My use case is to provide a way for a process to spawn a child and
>> > ensure that no descendants survive when that child dies.  Avoiding
>> > runaway processes is desirable in many situations.  My motivation is
>> > very lightweight (nested) sandboxing (every process is potentially
>> > sandboxed).
>> > 
>> > I.e., pid namespaces would be a pretty good fit (assuming they are
>> > sufficiently lightweight) but CLONE_NEWPID
>> 
>> sorry if this was already discussed, I didn't read this thread yet...
>> 
>> if CLONE_NEWPID is not suitable for any reason. We already have
>> PR_SET_CHILD_SUBREAPER. Perhaps we can simply add another
>> PR_SET_KILL_ALL_DESCEDANTS_ON_EXIT? we can use walk_process_tree()
>> to send SIGKILL.
>
> Yes, this is an option.  However, after the discussion in this thread I
> believe it would be better to drop the CAP_SYS_ADMIN requirement for
> CLONE_NEWPID (when no_new_privs is set) as this would avoid adding
> another API and code path for a similar effect.  I'm interested in
> possible security concerns about such a change.  Adding Andy Lutomirski
> to cc.

Absolutely not.  no_new_privs does not need the headache of being
increasing the kernel attack surface.

User namespaces are cheap, use one.  Let the people using no_new_privs
sleep easy.  We don't need to transform no_new_privs into a user namespace.

Eric