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    Subject[Patch] kernel/exit.c: Fix use-before-check in exit_mm()
    On Fri, Nov 16, 2007 at 01:34:54AM +0100, Jesper Juhl wrote:
    >On 13/11/2007, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
    >> Jesper Juhl wrote:
    >> > In kernel/exit.c we have this code :
    >> >
    >> > static void exit_mm(struct task_struct * tsk)
    >> > {
    >> > struct mm_struct *mm = tsk->mm;
    >> >
    >> > mm_release(tsk, mm);
    >> > if (!mm)
    >> > return;
    >> > ...
    >> >
    >> >
    >> > But, mm_release() may dereference it's second argument ('mm'), so
    >> > shouldn't we be doing the "!mm" test *before* we call mm_release() and
    >> > not after?
    >> > I don't know the mm code well enough to be able to tell if some of the
    >> > other stuff mm_release does needs to be done always and the mm
    >> > dereference can't actually happen, but maybe someone else who knows
    >> > the code better can tell... In any case, what's currently there looks
    >> > a little shaky..
    >> >
    >>
    >> Yeah, it looks wrong. mm_release() calls deactivate_mm() as its first
    >> act, which could well dereference mm (though it often doesn't).
    >>
    >So, whould simply moving the !mm check up as the first in the function
    >be an appropriate way to deal with this?

    I think yes. Patch below.

    Fix use-before-check in kernel/exit.c

    Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>

    ---

    diff --git a/kernel/exit.c b/kernel/exit.c
    index cd0f1d4..dca1e0d 100644
    --- a/kernel/exit.c
    +++ b/kernel/exit.c
    @@ -558,9 +558,9 @@ static void exit_mm(struct task_struct * tsk)
    {
    struct mm_struct *mm = tsk->mm;

    - mm_release(tsk, mm);
    if (!mm)
    return;
    + mm_release(tsk, mm);
    /*
    * Serialize with any possible pending coredump.
    * We must hold mmap_sem around checking core_waiters
    -
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