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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wjLMy+J20ZSBec4iarw2NeSu5sWXm6wdMH59n-e0Qe06g@mail.gmail.com>
Date:   Sat, 20 Mar 2021 10:56:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Jens Axboe <axboe@...nel.dk>, 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

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).

          Linus