Re: [PATCH v2 hmm 01/11] mm/hmm: fix use after free with struct hmm in the mmu notifiers

[Jason Gunthorpe <jgg-uk2M96/98Pc@public.gmane.org>]

On Thu, Jun 06, 2019 at 07:29:08PM -0700, John Hubbard wrote:
> On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> ...
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 8e7403f081f44a..547002f56a163d 100644
> > +++ b/mm/hmm.c
> ...
> > @@ -125,7 +130,7 @@ static void hmm_free(struct kref *kref)
> >  		mm->hmm = NULL;
> >  	spin_unlock(&mm->page_table_lock);
> >  
> > -	kfree(hmm);
> > +	mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
> 
> 
> It occurred to me to wonder if it is best to use the MMU notifier's
> instance of srcu, instead of creating a separate instance for HMM.

It *has* to be the MMU notifier SRCU because we are synchornizing
against the read side of that SRU inside the mmu notifier code, ie:

int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
        id = srcu_read_lock(&srcu);
        hlist_for_each_entry_rcu(mn, &range->mm->mmu_notifier_mm->list, hlist) {
                if (mn->ops->invalidate_range_start) {
                   ^^^^^

Here 'mn' is really hmm (hmm = container_of(mn, struct hmm,
mmu_notifier)), so we must protect the memory against free for the mmu
notifier core.

Thus we have no choice but to use its SRCU.

CH also pointed out a more elegant solution, which is to get the write
side of the mmap_sem during hmm_mirror_unregister - no notifier
callback can be running in this case. Then we delete the kref, srcu
and so forth.

This is much clearer/saner/better, but.. requries the callers of
hmm_mirror_unregister to be safe to get the mmap_sem write side.

I think this is true, so maybe this patch should be switched, what do
you think?

> > @@ -153,10 +158,14 @@ void hmm_mm_destroy(struct mm_struct *mm)
> >  
> >  static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> >  {
> > -	struct hmm *hmm = mm_get_hmm(mm);
> > +	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
> >  	struct hmm_mirror *mirror;
> >  	struct hmm_range *range;
> >  
> > +	/* hmm is in progress to free */
> 
> Well, sometimes, yes. :)

It think it is in all cases actually.. The only way we see a 0 kref
and still reach this code path is if another thread has alreay setup
the hmm_free in the call_srcu..

> Maybe this wording is clearer (if we need any comment at all):

I always find this hard.. This is a very standard pattern when working
with RCU - however in my experience few people actually know the RCU
patterns, and missing the _unless_zero is a common bug I find when
looking at code.

This is mm/ so I can drop it, what do you think?

Thanks,
Jason
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