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Date:   Mon, 21 Jun 2021 18:59:01 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
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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...

	There's a large mess around do_exit() - we have a bunch of
callers all over arch/*; if nothing else, I very much doubt that really
want to let tracer play with a thread in the middle of die_if_kernel()
or similar.

We sure as hell do not want to arrange for anything on the kernel
stack in such situations, no matter what's done in exit(2)...