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Date:   Thu, 14 Nov 2019 07:10:51 -0800
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Take slots_lock when using
 kvm_mmu_zap_all_fast()
Message-ID: <20191114151051.GB24045@linux.intel.com>
References: <20191113193032.12912-1-sean.j.christopherson@intel.com>
 <1b46d531-6423-3ccc-fc5f-df6fbaa02557@redhat.com>
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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.