On 25/04/17 19:49, Radim Krčmář 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()) ?
Doesn't [2/2] solve the exact same issue (that the release method cannot
be called twice in parallel)?
Not really. This could be a race between a release() and one of the other notifier
callbacks. e.g, In [0], we were hitting a use-after-free in kvm_unmap_hva() where,
the unregister could have succeeded and released the KVM.
[0] http://lkml.kernel.org/r/febea966-3767-21ff-3c40-1a76d1399138@xxxxxxx
In effect this all could be due to the same reason, the synchronize in unregister
missing another reader.
Suzuki
Thanks.
