Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced

[Pedro Alves]

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@xxxxxxxxxx> 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

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