Re: [PATCH 1/2] signal: don't allow sending any signals to PF_IO_WORKER threads

[Jens Axboe <axboe@kernel.dk>]

On 3/21/21 8:54 AM, Eric W. Biederman wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> On 3/20/21 3:38 PM, Eric W. Biederman wrote:
>>> Linus Torvalds <torvalds@linux-foundation.org> writes:
>>>
>>>> On Sat, Mar 20, 2021 at 9:19 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>>
>>>>> The creds should be reasonably in-sync with the rest of the threads.
>>>>
>>>> It's not about credentials (despite the -EPERM).
>>>>
>>>> It's about the fact that kernel threads cannot handle signals, and
>>>> then get caught in endless loops of "if (sigpending()) return
>>>> -EAGAIN".
>>>>
>>>> For a normal user thread, that "return -EAGAIN" (or whatever) will end
>>>> up returning an error to user space - and before it does that, it will
>>>> go through the "oh, returning to user space, so handle signal" path.
>>>> Which will clear sigpending etc.
>>>>
>>>> A thread that never returns to user space fundamentally cannot handle
>>>> this. The sigpending() stays on forever, the signal never gets
>>>> handled, the thread can't do anything.
>>>>
>>>> So delivering a signal to a kernel thread fundamentally cannot work
>>>> (although we do have some threads that explicitly see "oh, if I was
>>>> killed, I will exit" - think things like in-kernel nfsd etc).
>>>
>>> I agree that getting a kernel thread to receive a signal is quite
>>> tricky.  But that is not what the patch affects.
>>>
>>> The patch covers the case when instead of specifying the pid of the
>>> process to kill(2) someone specifies the tid of a thread.  Which implies
>>> that type is PIDTYPE_TGID, and in turn the signal is being placed on the
>>> t->signal->shared_pending queue.  Not the thread specific t->pending
>>> queue.
>>>
>>> So my question is since the signal is delivered to the process as a
>>> whole why do we care if someone specifies the tid of a kernel thread,
>>> rather than the tid of a userspace thread?
>>
>> Right, that's what this first patch does, and in all honesty, it's not
>> required like the 2/2 patch is. I do think it makes it more consistent,
>> though - the threads don't take signals, period. Allowing delivery from
>> eg kill(2) and then pass it to the owning task of the io_uring is
>> somewhat counterintuitive, and differs from earlier kernels where there
>> was no relationsship between that owning task and the async worker
>> thread.
>>
>> That's why I think the patch DOES make sense. These threads may share a
>> personality with the owning task, but I don't think we should be able to
>> manipulate them from userspace at all. That includes SIGSTOP, of course,
>> but also regular signals.
>>
>> Hence I do think we should do something like this.
> 
> I agree about signals.  Especially because being able to use kill(2)
> with the tid of thread is a linuxism and a backwards compatibility thing
> from before we had CLONE_THREAD.
> 
> I think for kill(2) we should just return -ESRCH.
> 
> Thank you for providing the reasoning that is what I really saw missing
> in the patches.  The why.  And software is difficult to maintain without
> the why.

Thanks Eric, I'll change that patch to -ESRCH and augment the commit
message a bit.

-- 
Jens Axboe