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

[Paolo Bonzini]

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?

Paolo