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

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Message-ID: <YNCaMDQVYB04bk3j@zeniv-ca.linux.org.uk>
Date:   Mon, 21 Jun 2021 13:54:56 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

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...