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

From: KOSAKI Motohiro
Date: Tue Nov 30 2010 - 21:50:41 EST


> 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@xxxxxxxxxxx>
> ---
> 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.


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