linux-kernel - [PATCH 02/24] KVM: x86/mmu: Check for shadow-present SPTE before querying A/D status

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Message-Id: <20210225204749.1512652-3-seanjc@google.com>
Date:   Thu, 25 Feb 2021 12:47:27 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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, Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 02/24] KVM: x86/mmu: Check for shadow-present SPTE before
 querying A/D status

When updating accessed and dirty bits, check that the new SPTE is present
before attempting to query its A/D bits.  Failure to confirm the SPTE is
present can theoretically cause a false negative, e.g. if a MMIO SPTE
replaces a "real" SPTE and somehow the PFNs magically match.

Realistically, this is all but guaranteed to be a benign bug.  Fix it up
primarily so that a future patch can tweak the MMU_WARN_ON checking A/D
status to fire if the SPTE is not-present.

Fixes: f8e144971c68 ("kvm: x86/mmu: Add access tracking for tdp_mmu")
Cc: Ben Gardon <bgardon@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c926c6b899a1..f46972892a2d 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -210,13 +210,12 @@ static int kvm_mmu_page_as_id(struct kvm_mmu_page *sp)
 
 static void handle_changed_spte_acc_track(u64 old_spte, u64 new_spte, int level)
 {
-	bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
-
 	if (!is_shadow_present_pte(old_spte) || !is_last_spte(old_spte, level))
 		return;
 
 	if (is_accessed_spte(old_spte) &&
-	    (!is_accessed_spte(new_spte) || pfn_changed))
+	    (!is_shadow_present_pte(new_spte) || !is_accessed_spte(new_spte) ||
+	     spte_to_pfn(old_spte) != spte_to_pfn(new_spte)))
 		kvm_set_pfn_accessed(spte_to_pfn(old_spte));
 }
 
@@ -444,7 +443,7 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 
 
 	if (was_leaf && is_dirty_spte(old_spte) &&
-	    (!is_dirty_spte(new_spte) || pfn_changed))
+	    (!is_present || !is_dirty_spte(new_spte) || pfn_changed))
 		kvm_set_pfn_dirty(spte_to_pfn(old_spte));
 
 	/*
-- 
2.30.1.766.gb4fecdf3b7-goog