linux-kernel - [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

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Message-Id: <20210622072454.3449146-1-seanjc@google.com>
Date:   Tue, 22 Jun 2021 00:24:54 -0700
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, David Matlack <dmatlack@...gle.com>
Subject: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
root.  KVM uses a "direct" PAE paging MMU when TDP is disabled and the
guest is running with paging disabled.  In that case, root_hpa points at
the pae_root page (of which only 32 bytes are used), not a standard
shadow page, and the WARN fires (a lot).

Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
Cc: David Matlack <dmatlack@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index b981a044ab55..1cae4485b3bc 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
 	if (WARN_ON(!VALID_PAGE(hpa)))
 		return false;
 
+	/*
+	 * A NULL shadow page is legal when shadowing a non-paging guest with
+	 * PAE paging, as the MMU will be direct with root_hpa pointing at the
+	 * pae_root page, not a shadow page.
+	 */
 	sp = to_shadow_page(hpa);
-	if (WARN_ON(!sp))
-		return false;
-
-	return is_tdp_mmu_page(sp) && sp->root_count;
+	return sp && is_tdp_mmu_page(sp) && sp->root_count;
 }
 #else
 static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
-- 
2.32.0.288.g62a8d224e6-goog