[PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY

[Zeng Chi]

From: Zeng Chi <zengchi@xxxxxxxxxx>

kvm_arch_prepare_memory_region() computes new->arch.flags, i.e. whether
a memslot is KVM_MEM_HUGEPAGE_CAPABLE or KVM_MEM_HUGEPAGE_INCAPABLE, only
for KVM_MR_CREATE and KVM_MR_MOVE, and returns early for every other
change.  But the generic code allocates a zeroed memslot for every
change and never copies old->arch, so after a KVM_MR_FLAGS_ONLY update,
e.g. toggling KVM_MEM_LOG_DIRTY_PAGES for live migration, the active
memslot has arch.flags == 0.

With both flags clear, fault_supports_huge_mapping() falls through to
the alignment check on the HVA range alone, which no longer verifies
that the GPA and HVA have the same offset within a PMD.  A memslot that
was marked KVM_MEM_HUGEPAGE_INCAPABLE because of a GPA/HVA offset
mismatch can then be mapped with PMD entries on read faults, and since
kvm_map_page() aligns the gfn and the pfn independently, the guest ends
up accessing the wrong host pages, exactly the "d -> f, e -> g" case
described in the comment above the check.

Carry the arch flags over from the old memslot for KVM_MR_FLAGS_ONLY,
as the GPA, HVA and size are guaranteed to be unchanged for that case.

Fixes: 7ab6fb505b2a ("LoongArch: KVM: Optimization for memslot hugepage checking")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zeng Chi <zengchi@xxxxxxxxxx>
---
 arch/loongarch/kvm/mmu.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
index e104897aa532..4ca7c6ea8f08 100644
--- a/arch/loongarch/kvm/mmu.c
+++ b/arch/loongarch/kvm/mmu.c
@@ -383,6 +383,16 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, const struct kvm_memory_slot
 	hva_t hva_start;
 	size_t size, gpa_offset, hva_offset;
 
+	/*
+	 * The generic code allocates a fresh, zeroed memslot for every change,
+	 * so the arch flags computed below must be carried over when only the
+	 * userspace flags change, e.g. when dirty logging is toggled.
+	 */
+	if (change == KVM_MR_FLAGS_ONLY) {
+		new->arch = old->arch;
+		return 0;
+	}
+
 	if ((change != KVM_MR_MOVE) && (change != KVM_MR_CREATE))
 		return 0;
 	/*
-- 
2.25.1


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