linux-kernel - Re: [PATCH] seccomp: Fix tracer exit notifications during fatal signals

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Message-ID: <CAGXu5j+Su0T4hN7kzi8s-YKM1X5=Aapgt-WMQg1yUka+HUQjPg@mail.gmail.com>
Date:   Tue, 23 Aug 2016 12:41:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kyle Huey <me@...ehuey.com>, James Morris <jmorris@...ei.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kyle Huey <khuey@...ehuey.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH] seccomp: Fix tracer exit notifications during fatal signals

On Mon, Aug 22, 2016 at 8:27 PM, Kyle Huey <me@...ehuey.com> wrote:
> On Thu, Aug 11, 2016 at 11:18 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Thu, Aug 11, 2016 at 8:12 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>>> On 08/10, Kees Cook wrote:
>>>>
>>>> This fixes a ptrace vs fatal pending signals bug as manifested in seccomp
>>>> now that ptrace was reordered to happen after ptrace. The short version is
>>>> that seccomp should not attempt to call do_exit() while fatal signals are
>>>> pending under a tracer. This was needlessly paranoid. Instead, the syscall
>>>> can just be skipped and normal signal handling, tracer notification, and
>>>> process death can happen.
>>>
>>> ACK.
>>>
>>> I think this change is fine in any case, but...
>>>
>>>> The bug happens because when __seccomp_filter() detects
>>>> fatal_signal_pending(), it calls do_exit() without dequeuing the fatal
>>>> signal. When do_exit() sends the PTRACE_EVENT_EXIT
>>>
>>> I _never_ understood what PTRACE_EVENT_EXIT should actually do. I mean,
>>> when it should actually stop. This was never defined.
>>
>> Yeah, agreed. I spent some time reading through what should happen to
>> __TASK_TRACED during exit and my head spun. :)
>>
>>>> notification and
>>>> that task is descheduled, __schedule() notices that there is a fatal
>>>> signal pending and changes its state from TASK_TRACED to TASK_RUNNING.
>>>
>>> And this can happen anyway, with or without this change, with or without
>>> seccomp. Because another fatal signal can be pending. So PTRACE_EVENT_EXIT
>>> actually depends on /dev/random.
>>>
>>> Perhaps we should finally define what it should do. Say, it should only
>>> stop if SIGKILL was sent "implicitely" by exit/exec. But as for exec,
>>> there are more (off-topic) complications, not sure we actually want this...
>>>
>>> Nevermind, the main problem is that _any_ change in this area can break
>>> something. This code is sooooooo old.
>>>
>>> But let me repeat, I think this change is fine anyway.
>>>
>>> Acked-by: Oleg Nesterov <oleg@...hat.com>
>>
>> Awesome, thanks!
>
> Hi folks,
>
> Can't help but notice this didn't make it into rc3.  Not sure if it's
> bubbling up somewhere I can't see, but we'd really like this to get
> into 4.8 so we don't have to work around the regression.
>
> Thanks!

Eek, thanks for the ping. I got distracted by some other bugs fixes to
get landed, but I've sent the seccomp pull request to James now. It
should show up for -rc4.

Thanks!

-Kees

-- 
Kees Cook
Nexus Security