Re: [PATCH v4 0/1] Use HMM for ODP v4

Jason Gunthorpe
2019-05-22T22:39:21Z

On Wed, May 22, 2019 at 06:04:20PM -0400, Jerome Glisse wrote:
> On Wed, May 22, 2019 at 05:12:47PM -0300, Jason Gunthorpe wrote:
> > On Wed, May 22, 2019 at 01:48:52PM -0400, Jerome Glisse wrote:
> >
> > > static void put_per_mm(struct ib_umem_odp *umem_odp)
> > > {
> > > struct ib_ucontext_per_mm *per_mm = umem_odp->per_mm;
> > > @@ -325,9 +283,10 @@ static void put_per_mm(struct ib_umem_odp *umem_odp)
> > > up_write(&per_mm->umem_rwsem);
> > >
> > > WARN_ON(!RB_EMPTY_ROOT(&per_mm->umem_tree.rb_root));
> > > - mmu_notifier_unregister_no_release(&per_mm->mn, per_mm->mm);
> > > + hmm_mirror_unregister(&per_mm->mirror);
> > > put_pid(per_mm->tgid);
> > > - mmu_notifier_call_srcu(&per_mm->rcu, free_per_mm);
> > > +
> > > + kfree(per_mm);
> >
> > Notice that mmu_notifier only uses SRCU to fence in-progress ops
> > callbacks, so I think hmm internally has the bug that this ODP
> > approach prevents.
> >
> > hmm should follow the same pattern ODP has and 'kfree_srcu' the hmm
> > struct, use container_of in the mmu_notifier callbacks, and use the
> > otherwise vestigal kref_get_unless_zero() to bail:
> >
> > From 0cb536dc0150ba964a1d655151d7b7a84d0f915a Mon Sep 17 00:00:00 2001
> > From: Jason Gunthorpe <jgg@mellanox.com>
> > Date: Wed, 22 May 2019 16:52:52 -0300
> > Subject: [PATCH] hmm: Fix use after free with struct hmm in the mmu notifiers
> >
> > mmu_notifier_unregister_no_release() is not a fence and the mmu_notifier
> > system will continue to reference hmm->mn until the srcu grace period
> > expires.
> >
> > CPU0 CPU1
> > __mmu_notifier_invalidate_range_start()
> > srcu_read_lock
> > hlist_for_each ()
> > // mn == hmm->mn
> > hmm_mirror_unregister()
> > hmm_put()
> > hmm_free()
> > mmu_notifier_unregister_no_release()
> > hlist_del_init_rcu(hmm-mn->list)
> > mn->ops->invalidate_range_start(mn, range);
> > mm_get_hmm()
> > mm->hmm = NULL;
> > kfree(hmm)
> > mutex_lock(&hmm->lock);
> >
> > Use SRCU to kfree the hmm memory so that the notifiers can rely on hmm
> > existing. Get the now-safe hmm struct through container_of and directly
> > check kref_get_unless_zero to lock it against free.
>
> It is already badly handled with BUG_ON()

You can't crash the kernel because userspace forced a race, and no it
isn't handled today because there is no RCU locking in mm_get_hmm nor
is there a kfree_rcu for the struct hmm to make the
kref_get_unless_zero work without use-after-free.

> i just need to convert those to return and to use
> mmu_notifier_call_srcu() to free hmm struct.

Isn't that what this patch does?

> The way race is avoided is because mm->hmm will either be NULL or
> point to another hmm struct before an existing hmm is free.

There is no locking on mm->hmm so it is useless to prevent races.

> Also if range_start/range_end use kref_get_unless_zero() but right
> now this is BUG_ON if it turn out to be NULL, it should just return
> on NULL.

Still needs rcu.

Also the container_of is necessary to avoid some race where you could
be doing:

CPU0 CPU1 CPU2
hlist_for_each ()
mmu_notifier_unregister_no_release(hmm1)
spin_lock(&mm->page_table_lock);
mm->hmm = NULL
spin_unlock(&mm->page_table_lock);
hmm2 = hmm_get_or_create()
mn == hmm1->mn
mn->ops->invalidate_range_start(mn, range)
mm_get_mm() == hmm2
hist_for_each con't
mn == hmm2->mn
mn->ops->invalidate_range_start(mn, range)
mm_get_mm() == hmm2

Now we called the same notifier twice on hmm2. Ooops.

There is no reason to risk this confusion just to avoid container_of.

So we agree this patch is necessary? Can you test it an ack it please?

Jason