linux-kernel - Re: [PATCH] KVM: x86/mmu: Take slots_lock when using kvm_mmu_zap_all

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Date:   Thu, 14 Nov 2019 13:16:21 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Take slots_lock when using
 kvm_mmu_zap_all_fast()

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