Re: [PATCH 1/2] kvm: Fix mmu_notifier release race

From: Suzuki K Poulose
Date: Wed May 03 2017 - 09:13:14 EST

On 28/04/17 18:20, Suzuki K Poulose wrote:
On 26/04/17 17:03, Suzuki K Poulose wrote:
On 25/04/17 19:49, Radim KrÄmÃÅ wrote:
2017-04-24 11:10+0100, Suzuki K Poulose:
The KVM uses mmu_notifier (wherever available) to keep track
of the changes to the mm of the guest. The guest shadow page
tables are released when the VM exits via mmu_notifier->ops.release().
There is a rare chance that the mmu_notifier->release could be
called more than once via two different paths, which could end
up in use-after-free of kvm instance (such as [0]).

e.g:

thread A thread B
------- --------------

get_signal-> kvm_destroy_vm()->
do_exit-> mmu_notifier_unregister->
exit_mm-> kvm_arch_flush_shadow_all()->
exit_mmap-> spin_lock(&kvm->mmu_lock)
mmu_notifier_release-> ....
kvm_arch_flush_shadow_all()-> .....
... spin_lock(&kvm->mmu_lock) .....
spin_unlock(&kvm->mmu_lock)
kvm_arch_free_kvm()
*** use after free of kvm ***

I don't understand this race ...
a piece of code in mmu_notifier_unregister() says:

/*
* Wait for any running method to finish, of course including
* ->release if it was run by mmu_notifier_release instead of us.
*/
synchronize_srcu(&srcu);

and code before that removes the notifier from the list, so it cannot be
called after we pass this point. mmu_notifier_release() does roughly
the same and explains it as:

/*
* synchronize_srcu here prevents mmu_notifier_release from returning to
* exit_mmap (which would proceed with freeing all pages in the mm)
* until the ->release method returns, if it was invoked by
* mmu_notifier_unregister.
*
* The mmu_notifier_mm can't go away from under us because one mm_count
* is held by exit_mmap.
*/
synchronize_srcu(&srcu);

The call of mmu_notifier->release is protected by srcu in both cases and
while it seems possible that mmu_notifier->release would be called
twice, I don't see a combination that could result in use-after-free
from mmu_notifier_release after mmu_notifier_unregister() has returned.

Thanks for bringing it up. Even I am wondering why this is triggered ! (But it
does get triggered for sure !!)

The only difference I can spot with _unregister & _release paths are the way
we use src_read_lock across the deletion of the entry from the list.

In mmu_notifier_unregister() we do :

id = srcu_read_lock(&srcu);
/*
* exit_mmap will block in mmu_notifier_release to guarantee
* that ->release is called before freeing the pages.
*/
if (mn->ops->release)
mn->ops->release(mn, mm);
srcu_read_unlock(&srcu, id);

## Releases the srcu lock here and then goes on to grab the spin_lock.

spin_lock(&mm->mmu_notifier_mm->lock);
/*
* Can not use list_del_rcu() since __mmu_notifier_release
* can delete it before we hold the lock.
*/
hlist_del_init_rcu(&mn->hlist);
spin_unlock(&mm->mmu_notifier_mm->lock);

While in mmu_notifier_release() we hold it until the node(s) are deleted from the
list :
/*
* SRCU here will block mmu_notifier_unregister until
* ->release returns.
*/
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist)
/*
* If ->release runs before mmu_notifier_unregister it must be
* handled, as it's the only way for the driver to flush all
* existing sptes and stop the driver from establishing any more
* sptes before all the pages in the mm are freed.
*/
if (mn->ops->release)
mn->ops->release(mn, mm);

spin_lock(&mm->mmu_notifier_mm->lock);
while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
mn = hlist_entry(mm->mmu_notifier_mm->list.first,
struct mmu_notifier,
hlist);
/*
* We arrived before mmu_notifier_unregister so
* mmu_notifier_unregister will do nothing other than to wait
* for ->release to finish and for mmu_notifier_unregister to
* return.
*/
hlist_del_init_rcu(&mn->hlist);
}
spin_unlock(&mm->mmu_notifier_mm->lock);
srcu_read_unlock(&srcu, id);

## The lock is release only after the deletion of the node.

Both are followed by a synchronize_srcu(). Now, I am wondering if the unregister path
could potentially miss SRCU read lock held in _release() path and go onto finish the
synchronize_srcu before the item is deleted ? May be we should do the read_unlock
after the deletion of the node in _unregister (like we do in the _release()) ?

I haven't been able to reproduce the mmu_notifier race condition, which leads to KVM
free, reported at [1]. I will leave it running (with tracepoints/ftrace) over the
weekend.


I couldn't reproduce the proposed "mmu_notifier race" reported in [0].
However I found some other use-after-free cases in the unmap_stage2_range()
code due to the introduction of cond_resched_lock(). It may be just that the
IP reported in [0] was for wrong line of code ? i.e, arch_spin_is_locked instead
of unmap_stage2_range ?
Anyways, I will send a new version of the patches in a separate series.

[0] https://marc.info/?l=linux-kernel&m=149201399018791&w=2

Suzuki