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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 21 Jun 2021 16:36:33 -0700
Message-ID: <CAHk-=wjtagi3g5thA-T8ooM8AXcy3brdHzugCPU0itdbpDYH_A@mail.gmail.com>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Michael Schmitz <schmitzmic@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        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>, Kees Cook <keescook@chromium.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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On Mon, Jun 21, 2021 at 4:23 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
>         How would it help e.g. oopsen on the way out of timer interrupts?
> IMO we simply shouldn't allow ptrace access if the tracee is in that kind
> of state, on any architecture...

Yeah no, we can't do the "wait for ptrace" when the exit is due to an
oops. Although honestly, we have other cases like that where do_exit()
isn't 100% robust if you kill something in an interrupt. Like all the
locks it leaves locked etc.

So do_exit() from a timer interrupt is going to cause problems
regardless. I agree it's probably a good idea to try to avoid causing
even more with the odd ptrace thing, but I don't think ptrace_event is
some really "fundamental" problem at that point - it's just one detail
among many many.

So I was more thinking of the debug patch for m68k to catch all the
_regular_ cases, and all the other random cases of ptrace_event() or
ptrace_notify().

Although maybe we've really caught them all. The exit case was clearly
missing, and the thread fork case was scrogged. There are patches for
the known problems. The patches I really don't like are the
verification ones to find any unknown ones..

            Linus