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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 21 Jun 2021 08:38:14 -0700
Message-ID: <CAHk-=wjC_=iwj61vSZ0orPGjrxhDK3Ob2D4eaRpXXFdAbF6coQ@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>
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On Mon, Jun 21, 2021 at 6:55 AM Al Viro <viro@zeniv.linux.org.uk> 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...

Hmm. Maybe the alternatives would all be worse. The current thing is
clever, and shares the return path with the normal case. It's just
also a bit surprising, in that a kernel thread normally must not
return - with the magical exception of "if it had done a
kernel_execve() at some point, then returning is magically the way you
actually start user mode".

So it all feels very special, and there's not even a comment about it.

I think we only have two users of that thing (the very first 'init',
and user-mode-helpr), So I guess it doesn't really matter.

            Linus