Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 30 Nov 2010 16:09:50 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] mm_release: Do a set_fs(USER_DS) before handling clear_child_tid. |
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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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