[Patch] kernel/exit.c: Fix use-before-check in exit_mm()

[WANG Cong <xiyou.wangcong@gmail.com>]

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