Re: [PATCH v2] do_exit(): Make sure we run with get_fs() == USER_DS.

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not
> otherwise reset before do_exit(). do_exit may later (via mm_release in fork.c)
> do a put_user to a user-controlled address, potentially allowing a user to
> leverage an oops into a controlled write into kernel memory.
> 
> A more logical place to put this might be when we know an oops has occurred,
> before we call do_exit(), but that would involve changing every architecture, in
> multiple places. Let's just stick it in do_exit instead.
> 
> Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
> ---
>  kernel/exit.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 21aa7b3..68899b3 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -914,6 +914,14 @@ NORET_TYPE void do_exit(long code)
>  	if (unlikely(!tsk->pid))
>  		panic("Attempted to kill the idle task!");
>  
> +	/*
> +	 * 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. This is relevant at least for clearing clear_child_tid in
> +	 * mm_release.
> +	 */
> +	set_fs(USER_DS);

"This is relevant" is no good explanation ;)
Please recognize this is tricky code and Please consider to write more 
careful and looooong comments.

Thanks.