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

[John Hubbard <jhubbard@nvidia.com>]

On 6/7/19 5:34 AM, Jason Gunthorpe wrote:
> 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.

Ah right. It's embarassingly obvious when you say it out loud. :) 
Thanks for explaining.

> 
> 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?

OK, your follow-up notes that we'll leave it as is, got it.


thanks,
-- 
John Hubbard
NVIDIA