linux-kernel - Re: Kernel stack read with PTRACE_EVENT_EXIT and io

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Date:   Mon, 21 Jun 2021 16:36:33 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michael Schmitz <schmitzmic@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Tejun Heo <tj@...nel.org>, Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

On Mon, Jun 21, 2021 at 4:23 PM Al Viro <viro@...iv.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