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