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SubjectRe: [PATCH] mm_release: Do a set_fs(USER_DS) before handling clear_child_tid.
On Mon, 29 Nov 2010 21:19:16 -0500
Nelson Elhage <nelhage@ksplice.com> wrote:

> If a user manages to trigger a kernel BUG() or page fault with fs set to
> KERNEL_DS, fs is not otherwise reset before do_exit(), allowing the user to
> write a 0 to an arbitrary address in kernel memory.
>
> Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
> ---
> AFAICT this is presently only triggerable in the presence of another bug, but
> this potentially turns a lot of DoS bugs into privilege escalation, so it's
> worth fixing. Among other things, sock_no_sendpage and the kernel_{read,write}v
> calls in splice.c make it easy to call an awful lot of the kernel under
> KERNEL_DS.
>
> This isn't the only way we could fix this -- we could put the set_fs() at the
> start of do_exit, or in all the callers that might call potentially do_exit with
> KERNEL_DS set, or else we could do an access_ok inside fork(). I'm happy to put
> together one of those patches if someone thinks another approach makes more
> sense.
>
> kernel/fork.c | 5 +++++
> 1 files changed, 5 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 3b159c5..a68445e 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -636,7 +636,12 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
> /*
> * We don't check the error code - if userspace has
> * not set up a proper pointer then tough luck.
> + *
> + * We do set_fs() explicitly in case this task
> + * exited while inside set_fs(KERNEL_DS) for
> + * some reason (e.g. on a BUG()).
> */
> + set_fs(USER_DS);
> put_user(0, tsk->clear_child_tid);
> sys_futex(tsk->clear_child_tid, FUTEX_WAKE,
> 1, NULL, NULL, 0);

Confused. The user can only exploit the wrong addr_limit if control
returns to userspace for the user's code to execute. But that won't be
happening, because this thread will unconditionally exit.


If/when you unconfuse me, I'd suggest this change only be done if the
thread is *known* to have oopsed - doing it for non-oopsed threads
seems unpleasant to my mind. And I think it should be done nice and
clearly, right up inside do_exit() by some means. Or perhaps in the
oops code, just before it calls do_exit(). Not hidden down in
mm_release().


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