This is the mail archive of the
gdb@sourceware.org
mailing list for the GDB project.
Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced
- From: Pedro Alves <palves at redhat dot com>
- To: Oleg Nesterov <oleg at redhat dot com>, Denys Vlasenko <vda dot linux at googlemail dot com>
- Cc: Denys Vlasenko <dvlasenk at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Dmitry Vyukov <dvyukov at google dot com>, Alexander Potapenko <glider at google dot com>, Eric Dumazet <edumazet at google dot com>, Jan Kratochvil <jan dot kratochvil at redhat dot com>, Julien Tinnes <jln at google dot com>, Kees Cook <keescook at google dot com>, Kostya Serebryany <kcc at google dot com>, Linus Torvalds <torvalds at linux-foundation dot org>, "Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>, Robert Swiecki <swiecki at google dot com>, Roland McGrath <roland at hack dot frob dot com>, syzkaller at googlegroups dot com, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, "gdb at sourceware dot org" <gdb at sourceware dot org>
- Date: Mon, 26 Oct 2015 12:08:56 +0000
- Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced
- Authentication-results: sourceware.org; auth=none
- References: <20151020171740 dot GA29290 at redhat dot com> <20151020171754 dot GA29304 at redhat dot com> <20151020153155 dot e03f4219da4014efe6f810b0 at linux-foundation dot org> <5627EE9E dot 8040600 at redhat dot com> <5627F607 dot 4050506 at redhat dot com> <20151021214703 dot GA1810 at redhat dot com> <CAK1hOcP027FAWg5uVDjiFDC+0=dGu5JFJcy9Jij4dY8SBnT4MA at mail dot gmail dot com> <20151025155440 dot GB2043 at redhat dot com>
On 10/25/2015 03:54 PM, Oleg Nesterov wrote:
> On 10/22, Denys Vlasenko wrote:
>>
>> On Wed, Oct 21, 2015 at 11:47 PM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 10/21, Denys Vlasenko wrote:
>>>>
>>>> On 10/21/2015 09:59 PM, Denys Vlasenko wrote:
>>>>> On 10/21/2015 12:31 AM, Andrew Morton wrote:
>>>>>> Well, to fix this a distro needs to roll out a new kernel. Or a new
>>>>>> init(8). Is there any reason to believe that distributing/deploying a
>>>>>> new kernel is significantly easier for everyone? Because fixing init
>>>>>> sounds like a much preferable solution to this problem.
>>>>>
>>>>> People will continue to write new init(8) implementations,
>>>>> and they will miss this obscure case.
>>>>>
>>>>> Before this bug was found, it was considered possible to use
>>>>> a shell script as init process. What now, every shell needs to add
>>>>> __WALL to its waitpids?
>>>
>>> Why not? I think it can safely use __WALL too.
>>
>> Because having any userspace program which can happen to be init,
>> which includes all shells out there in the wild
>> (bash, dash, ash, ksh, zsh, msh, hush,...)
>> learn about __WALL is wrong: apart from this wart, they do not have
>> to use any Linux-specific code. It can all be perfectly legitimate POSIX.
>
> Yes, this is true. I meant that they could safely use __WALL to, but I
> understand that this change can be painful.
>
>>> Sure. But people do the things which were never intended to be
>>> used all the time. We simply can not know if this "feature"
>>> already has a creative user or not.
>>
>> It won't be unfixable: they will just have to switch from PTRACE_TRACEME
>> to PTRACE_ATTACH.
>>
>> As of now we do not know any people craz^W creative enough
>> to create a cross between init and strace. If such specimens would
>> materialize, don't they deserve to have to make that change?
>
> This also applies to people who use bash/whatever as /sbin/init and allow
> the untrusted users to run the exploits ;) I do not know who is more crazy.
>
> 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, and it's not unreasonable to assume
that e.g., a program asserts that waitpid either returns error or a
known (process) PID.
That's not an init-only issue, but something that might affect any
process that runs a child that happens to decide to
call PTRACE_TRACEME.
The ptrace man page says:
"A process can initiate a trace by calling fork(2) and having the resulting
child do a PTRACE_TRACEME, followed (typically) by an execve(2)."
Given that, can we instead make the kernel error out on PTRACE_TRACEME issued
from a non-leader thread? Then between PTRACE_TRACEME and the parent's
waitpid, __WALL or !__WALL should make no difference.
(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,
I believe). All this because PTRACE_TRACEME is broken by design by making it
be the child's choice whether to be traced...)
Thanks,
Pedro Alves