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

[Sean Christopherson <sean.j.christopherson@intel.com>]

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.