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

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Date:   Wed, 23 Jun 2021 09:33:50 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        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>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, Jun 22, 2021 at 1:53 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> Playing with it some more I think I have everything working working
>> except for PTRACE_EVENT_SECCOMP (which can stay ptrace_event) and
>> group_exit(2).
>>
>> Basically in exit sending yourself a signal and then calling do_exit
>> from the signal handler is not unreasonable, as exit is an ordinary
>> system call.
>
> Ok, this is a bit odd, but I do like the concept of just making
> ptrace_event just post a signal, and have all ptrace things always be
> handled at signal time (or the special system call entry/exit, which
> is fine too).
>
>> For purposes of discussion this is my current draft implementation.
>
> I didn't check what is so different about exit_group() that you left
> that as an exercise for the reader, but if that ends up then removing
> the whole "wait synchromously for ptrace" cases for good I don't
> _hate_ this. It's a bit odd, but it would be really nice to limit
> where ptrace picks up data.

I am still figuring out exit_group.  I am hoping for sometime today.
My intuition tells me I can do it, and I have a sense of what threads I
need to pull to get there.  I just don't know what the code is going to
look like yet.

Basically solving exit_group means moving ptrace_event out of do_exit.

> We do end up doing that stuff in "get_signal()", and that means that
> we have the interaction with io_uring calling it directly, but it's at
> least not a new thing.

The ugliest bit is having to repeat the wait_for_vfork_done both in fork
and in get_signal.

Eric