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Message-ID: <f732440f-bfad-f2f8-2524-037f03f31c10@arm.com>
Date: Wed, 3 May 2017 14:13:01 +0100
From: Suzuki K Poulose <Suzuki.Poulose@....com>
To: Radim Krčmář <rkrcmar@...hat.com>
Cc: kvm@...r.kernel.org, marc.zyngier@....com, andreyknvl@...gle.com,
Will Deacon <Will.Deacon@....com>,
linux-kernel@...r.kernel.org, pbonzini@...hat.com,
paulmck@...ux.vnet.ibm.com, kvmarm@...ts.cs.columbia.edu,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] kvm: Fix mmu_notifier release race
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 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()) ?
>
> 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
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