Messages in this thread Patch in this message |  | | | Date | Wed, 1 Dec 2010 17:12:05 -0800 | | From | Andrew Morton <> | | Subject | Re: [PATCH v2] do_exit(): Make sure we run with get_fs() == USER_DS. |
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On Tue, 30 Nov 2010 21:27:36 -0500 Nelson Elhage <nelhage@ksplice.com> wrote:
> 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); > + > tracehook_report_exit(&code); > > validate_creds_for_do_exit(tsk);
I think that the potential of escalating an oops or a BUG into a local root hole is pretty serious so I'll send this fix along for 2.6.37 and I tagged it for -stable backporting, along with a sterner-sounding changelog.
From: Nelson Elhage <nelhage@ksplice.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.
This is only triggerable in the presence of another bug, but this potentially turns a lot of DoS bugs into privilege escalations, so it's worth fixing. I have proof-of-concept code which uses this bug along with CVE-2010-3849 to write a zero to an arbitrary kernel address, so I've tested that this is not theoretical.
A more logical place to put this fix 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.
[akpm@linux-foundation.org: update code comment] Signed-off-by: Nelson Elhage <nelhage@ksplice.com> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> ---
kernel/exit.c | 9 +++++++++ 1 file changed, 9 insertions(+)
diff -puN kernel/exit.c~do_exit-make-sure-we-run-with-get_fs-==-user_ds kernel/exit.c --- a/kernel/exit.c~do_exit-make-sure-we-run-with-get_fs-==-user_ds +++ a/kernel/exit.c @@ -914,6 +914,15 @@ 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. Amongst other possible reasons, this is to prevent + * mm_release()->clear_child_tid() from writing to a user-controlled + * kernel address. + */ + set_fs(USER_DS); + tracehook_report_exit(&code); validate_creds_for_do_exit(tsk); _
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