linux-kernel - Re: mm_release() call in exit

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Date:	Fri, 16 Nov 2007 01:34:54 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: mm_release() call in exit_mm() looks dangerous

On 13/11/2007, Jeremy Fitzhardinge <jeremy@...p.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?

-- 
Jesper Juhl <jesper.juhl@...il.com>
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