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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Jun 2021 15:04:33 -0700
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Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        alpha <linux-alpha@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>,
        Daniel Jacobowitz <drow@nevyn.them.org>,
        Kees Cook <keescook@chromium.org>
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On Thu, Jun 10, 2021 at 1:58 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> The problem is sometimes we read all of the registers from
> a context where they are not all saved.

Ouch. Yes. And this is really painful because none of the *normal*
architectures do this, so it gets absolutely no coverage.

> I think at this point we need to say that the architectures that have a
> do this need to be fixed to at least call do_exit and the kernel
> function in create_io_thread with the deeper stack.

Yeah. We traditionally have that requirement for fork() and friends
too (vfork/clone), so adding exit and io_uring to do so seems like the
most straightforward thing.

But I really wish we had some way to test and trigger this so that we
wouldn't get caught on this before. Something in task_pt_regs() that
catches "this doesn't actually work" and does a WARN_ON_ONCE() on the
affected architectures?

               Linus