Re: [PATCH] KVM: x86/mmu: Take slots_lock when using kvm_mmu_zap_all_fast()

[Paolo Bonzini]

On 14/11/19 16:10, Sean Christopherson wrote:
> On Thu, Nov 14, 2019 at 01:16:21PM +0100, Paolo Bonzini wrote:
>> On 13/11/19 20:30, Sean Christopherson wrote:
>>> Failing to take slots_lock when toggling nx_huge_pages allows multiple
>>> instances of kvm_mmu_zap_all_fast() to run concurrently, as the other
>>> user, KVM_SET_USER_MEMORY_REGION, does not take the global kvm_lock.
>>> Concurrent fast zap instances causes obsolete shadow pages to be
>>> incorrectly identified as valid due to the single bit generation number
>>> wrapping, which results in stale shadow pages being left in KVM's MMU
>>> and leads to all sorts of undesirable behavior.
>>
>> Indeed the current code fails lockdep miserably, but isn't the whole
>> body of kvm_mmu_zap_all_fast() covered by kvm->mmu_lock?  What kind of
>> badness can happen if kvm->slots_lock isn't taken?
> 
> kvm_zap_obsolete_pages() temporarily drops mmu_lock and reschedules so
> that it doesn't block other vCPUS from inserting shadow pages into the new
> generation of the mmu.

Oh, of course.  I've worked on all this on the pre-5.4 MMU and that does
not have commit ca333add693 ("KVM: x86/mmu: Explicitly track only a
single invalid mmu generation").

I queued this patch with a small tweak to the commit message, to explain
why it doesn't need a stable backport.

Paolo