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

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Date:   Mon, 21 Jun 2021 11:50:56 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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

Al Viro <viro@...iv.linux.org.uk> writes:

> On Mon, Jun 21, 2021 at 01:54:56PM +0000, Al Viro wrote:
>> On Tue, Jun 15, 2021 at 02:58:12PM -0700, Linus Torvalds wrote:
>> 
>> > And I think our horrible "kernel threads return to user space when
>> > done" is absolutely horrifically nasty. Maybe of the clever sort, but
>> > mostly of the historical horror sort.
>> 
>> How would you prefer to handle that, then?  Separate magical path from
>> kernel_execve() to switch to userland?  We used to have something of
>> that sort, and that had been a real horror...
>> 
>> As it is, it's "kernel thread is spawned at the point similar to
>> ret_from_fork(), runs the payload (which almost never returns) and
>> then proceeds out to userland, same way fork(2) would've done."
>> That way kernel_execve() doesn't have to do anything magical.
>> 
>> Al, digging through the old notes and current call graph...
>
> 	FWIW, the major assumption back then had been that get_signal(),
> signal_delivered() and all associated machinery (including coredumps)
> runs *only* from SIGPENDING/NOTIFY_SIGNAL handling.
>
> 	And "has complete registers on stack" is only a part of that;
> there was other fun stuff in the area ;-/  Do we want coredumps for
> those, and if we do, will the de_thread stuff work there?

Do we want coredumps from processes that use io_uring? yes
Exactly what we want from io_uring threads is less clear.  We can't
really give much that is meaningful beyond the thread ids of the
io_uring threads.

What problems do are you seeing beyond the missing registers on the
stack for kernel threads?

I don't immediately see the connection between coredumps and de_thread.

The function de_thread arranges for the fatal_signal_pending to be true,
and that should work just fine for io_uring threads.  The io_uring
threads process the fatal_signal with get_signal and then proceed to
exit eventually calling do_exit.

Eric