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

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Message-ID: <20191114151051.GB24045@linux.intel.com>
Date:   Thu, 14 Nov 2019 07:10:51 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        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 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.