linux-kernel - Re: [PATCH 1/2] signal: don't allow sending any signals to PF_IO

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Date:   Sun, 21 Mar 2021 09:54:00 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        io-uring <io-uring@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Stefan Metzmacher <metze@...ba.org>
Subject: Re: [PATCH 1/2] signal: don't allow sending any signals to PF_IO_WORKER threads

Jens Axboe <axboe@...nel.dk> writes:

> On 3/20/21 3:38 PM, Eric W. Biederman wrote:
>> Linus Torvalds <torvalds@...ux-foundation.org> writes:
>> 
>>> On Sat, Mar 20, 2021 at 9:19 AM Eric W. Biederman <ebiederm@...ssion.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.





Eric