Re: [PATCH 03/10] exit: Move oops specific logic from do_exit into make_task_dead

Eric W. Biederman
2022-01-07T19:01:43Z

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

> On Wed, Dec 08, 2021 at 02:25:25PM -0600, Eric W. Biederman wrote:
>> - /*
>> - * 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...

Thank you for catching that. I was leaning too much on the description
in the comment of why force_uaccess_begin is there.

Catching up on the state of set_fs/get_fs removal it appears like a lot
of progress has been made and on a lot of architectures set_fs/get_fs is
just gone, and force_uaccess_begin is a noop.

On architectures that still have set_fs/get_fs it appears all of the old
warts are present and kernel threads still run with set_fs(KERNEL_DS).

Assuming it won't be too much longer before the rest of the arches have
set_fs/get_fs removed it looks like it makes sense to leave the
force_uaccess_begin where it is, and just let force_uaccess_begin be
removed when set_fs/get_fs are removed from the tree.

Christoph does it look like the set_fs/get_fs removal work is going
to stall indefinitely on some architectures? If so I think we want to
find a way to get kernel threads to run with set_fs(USER_DS) on the
stalled architectures. Otherwise I think we have a real hazard of
introducing bugs that will only show up on the stalled architectures.

I finally understand now why when I updated set_child_tid in the kthread
code early in fork why x86 was fine another architecture was not.

Eric