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Date:   Wed, 18 Jan 2023 10:43:19 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH 26/41] kernel/fork: assert no VMA readers during its
 destruction
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On Tue 17-01-23 17:53:00, Suren Baghdasaryan wrote:
> On Tue, Jan 17, 2023 at 7:42 AM 'Michal Hocko' via kernel-team
> <kernel-team@android.com> wrote:
> >
> > On Mon 09-01-23 12:53:21, Suren Baghdasaryan wrote:
> > > Assert there are no holders of VMA lock for reading when it is about to be
> > > destroyed.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  include/linux/mm.h | 8 ++++++++
> > >  kernel/fork.c      | 2 ++
> > >  2 files changed, 10 insertions(+)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 594e835bad9c..c464fc8a514c 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -680,6 +680,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> > >       VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> > >  }
> > >
> > > +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> > > +{
> > > +     VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> > > +                   vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> > > +                   vma);
> >
> > Do we really need to check for vm_lock_seq? rwsem_is_locked should tell
> > us something is wrong on its own, no? This could be somebody racing with
> > the vma destruction and using the write lock. Unlikely but I do not see
> > why to narrow debugging scope.
> 
> I wanted to ensure there are no page fault handlers (read-lockers)
> when we are destroying the VMA and rwsem_is_locked(&vma->lock) alone
> could trigger if someone is concurrently calling vma_write_lock(). But
> I don't think we expect someone to be write-locking the VMA while we

That would be UAF, no?

> are destroying it, so you are right, I'm overcomplicating things here.
> I think I can get rid of vma_assert_no_reader() and add
> VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock)) directly in
> __vm_area_free(). WDYT?

Yes, that adds some debugging. Not sure it is really necessary buyt it
is VM_BUG_ON so why not.
-- 
Michal Hocko
SUSE Labs