linux-kernel - [PATCH v2 11/30] KVM: x86/mmu: Check for !leaf=>leaf, not PFN change, in TDP MMU SP removal

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Date:   Thu, 23 Dec 2021 22:22:59 +0000
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>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: [PATCH v2 11/30] KVM: x86/mmu: Check for !leaf=>leaf, not PFN change,
 in TDP MMU SP removal

Look for a !leaf=>leaf conversion instead of a PFN change when checking
if a SPTE change removed a TDP MMU shadow page.  Convert the PFN check
into a WARN, as KVM should never change the PFN of a shadow page (except
when its being zapped or replaced).

>From a purely theoretical perspective, it's not illegal to replace a SP
with a hugepage pointing at the same PFN.  In practice, it's impossible
as that would require mapping guest memory overtop a kernel-allocated SP.
Either way, the check is odd.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 3b13249bbbe1..05f35541ff2f 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -518,9 +518,12 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 
 	/*
 	 * Recursively handle child PTs if the change removed a subtree from
-	 * the paging structure.
+	 * the paging structure.  Note the WARN on the PFN changing without the
+	 * SPTE being converted to a hugepage (leaf) or being zapped.  Shadow
+	 * pages are kernel allocations and should never be migrated.
 	 */
-	if (was_present && !was_leaf && (pfn_changed || !is_present))
+	if (was_present && !was_leaf &&
+	    (is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
 		handle_removed_tdp_mmu_page(kvm,
 				spte_to_child_pt(old_spte, level), shared);
 }
-- 
2.34.1.448.ga2b2bfdf31-goog