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

[Linus Torvalds <torvalds@linux-foundation.org>]

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).

          Linus