linux-kernel - Re: [PATCH 03/10] exit: Move oops specific logic from do_exit into make_task

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Message-ID: <YdUxGKRcSiDy8jGg@zeniv-ca.linux.org.uk>
Date:   Wed, 5 Jan 2022 05:48:08 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Gladkov <legion@...nel.org>,
        Kyle Huey <me@...ehuey.com>, Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 03/10] exit: Move oops specific logic from do_exit into
 make_task_dead

On Wed, Dec 08, 2021 at 02:25:25PM -0600, Eric W. Biederman wrote:
> The beginning of do_exit has become cluttered and difficult to read as
> it is filled with checks to handle things that can only happen when
> the kernel is operating improperly.
> 
> Now that we have a dedicated function for cleaning up a task when the
> kernel is operating improperly move the checks there.

Umm...  I would probably take profile_task_exit() crap out before that
point.
	1) the damn thing is dead - nothing registers notifiers there
	2) blocking_notifier_call_chain() is not a nice thing to do on oops...

I'll post a patch ripping the dead parts of kernel/profile.c out tomorrow
morning (there's also profile_handoff_task(), equally useless these days
and complicating things for __put_task_struct()).

> -	/*
> -	 * If do_exit is called because this processes oopsed, it's possible
> -	 * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
> -	 * continuing. Amongst other possible reasons, this is to prevent
> -	 * mm_release()->clear_child_tid() from writing to a user-controlled
> -	 * kernel address.
> -	 */
> -	force_uaccess_begin();

Are you sure about that one?  It shouldn't matter, but... it's a potential
change for do_exit() from a kernel thread.  As it is, we have that
force_uaccess_begin() for exiting threads and for kernel ones it's not
a no-op.  I'm not concerned about attempted userland access after that
point for those, obviously, but I'm not sure you won't step into something
subtle here.

I would prefer to split that particular change off into a separate commit...