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

[Kees Cook]

On Thu, Aug 11, 2016 at 8:12 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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@xxxxxxxxxx>

Awesome, thanks!

-Kees

-- 
Kees Cook
Nexus Security