linux-kernel - Re: [PATCH 1/2] wait/ptrace: always assume _

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Message-ID: <5630ED16.50900@redhat.com>
Date:	Wed, 28 Oct 2015 15:43:18 +0000
From:	Pedro Alves <palves@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Julien Tinnes <jln@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"gdb@...rceware.org" <gdb@...rceware.org>
Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is
 traced

On 10/28/2015 04:11 PM, Oleg Nesterov wrote:
> On 10/26, Pedro Alves wrote:
>>
>> On 10/25/2015 03:54 PM, Oleg Nesterov wrote:
>>>
>>> In any case, the real question is whether we should change the kernel to
>>> fix the problem, or ask the distros to fix their init's. In the former
>>> case 1/2 looks simpler/safer to me than the change in ptrace_traceme(),
>>> and you seem to agree that 1/2 is not that bad.
>>
>> A risk here seems to be that waitpid will start returning unexpected
>> (thread) PIDs to parent processes,
> 
> I don't see how this change can make the things worse,
> 
>> and it's not unreasonable to assume
>> that e.g., a program asserts that waitpid either returns error or a
>> known (process) PID.
> 
> Well. /sbin/init can never assume this, obviously.

Right.  I was actually thinking of !init processes -- basically code
that spawns helper processes, keeps a data structure indexed by pid, then
discards the structure when the child exits.  Something like:

 pid = waitpid(-1, &status, 0);
 if (pid > 0)
 {
   struct child_process *child = find_process(pid);
   assert (child != NULL);
 }

As in, before your change, the child could get stuck forever, but after your
change, the parent could die/assert instead.

But ...

> 
>> That's not an init-only issue,
> 
> Yes. Because we have CLONE_PARENT. So "waitpid either returns error or a
> known (process) PID" is only true if you trust your children.

... OK, that's indeed a good point.

>> (Also, in the original test case, if the child gets/raises a signal or execs
>> before exiting, the bash/init/whatever process won't be issuing PTRACE_CONT,
>> and the child will thus end up stuck (though should be SIGKILLable,
>
> Oh, but if it is killable everything is fine. How does this differ from the
> case when, say, you jusr reparent to init and do kill(getpid(), SIGSTOP) ?

The difference is that if the child called PTRACE_TRACEME, then it goes
to ptrace-stop instead and no amount of SIGCONT unstucks it -- the only way
out is force killing.  I agree it's not a major issue as there's a way out
(and thus made it a parens), but I wouldn't call it nice either.

>> All this because PTRACE_TRACEME is broken by design
>
> Heh. I agree. But we can't fix it now.

Perhaps the man page could document it as deprecated, suggesting
PTRACE_ATTACH/PTRACE_SEIZE instead?

Thanks,
Pedro Alves

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