[PATCH 26/41] kernel/fork: assert no VMA readers during its destruction

[Suren Baghdasaryan]

On Wed, Jan 18, 2023 at 1:43 AM 'Michal Hocko' via kernel-team
<kernel-team at android.com> wrote:
>
> 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 at 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 at 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?

Yes. That's why what I have is an overkill (which is also racy).

>
> > 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.

I would like to keep it if possible. If it triggers that would be a
clear signal what the issue is. Otherwise it might be hard to debug
such a corner case.

> --
> Michal Hocko
> SUSE Labs
>
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