KVM: x86/mmu: fix potential races when walking host page table

KVM uses lookup_address_in_mm() to detect the hugepage size that the host
uses to map a pfn.  The function suffers from several issues:

 - no usage of READ_ONCE(*). This allows multiple dereference of the same
   page table entry. The TOCTOU problem because of that may cause KVM to
   incorrectly treat a newly generated leaf entry as a nonleaf one, and
   dereference the content by using its pfn value.

 - the information returned does not match what KVM needs; for non-present
   entries it returns the level at which the walk was terminated, as long
   as the entry is not 'none'.  KVM needs level information of only 'present'
   entries, otherwise it may regard a non-present PXE entry as a present
   large page mapping.

 - the function is not safe for mappings that can be torn down, because it
   does not disable IRQs and because it returns a PTE pointer which is never
   safe to dereference after the function returns.

So implement the logic for walking host page tables directly in KVM, and
stop using lookup_address_in_mm().

Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Message-Id: <20220429031757.2042406-1-mizhang@google.com>
[Inline in host_pfn_mapping_level, ensure no semantic change for its
 callers. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -2804,8 +2804,12 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 				  const struct kvm_memory_slot *slot)
 {
 	unsigned long hva;
-	pte_t *pte;
-	int level;
+	unsigned long flags;
+	int level = PG_LEVEL_4K;
+	pgd_t pgd;
+	p4d_t p4d;
+	pud_t pud;
+	pmd_t pmd;
 
 	if (!PageCompound(pfn_to_page(pfn)) && !kvm_is_zone_device_pfn(pfn))
 		return PG_LEVEL_4K;
@@ -2820,10 +2824,43 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	 */
 	hva = __gfn_to_hva_memslot(slot, gfn);
 
-	pte = lookup_address_in_mm(kvm->mm, hva, &level);
-	if (unlikely(!pte))
-		return PG_LEVEL_4K;
+	/*
+	 * Lookup the mapping level in the current mm.  The information
+	 * may become stale soon, but it is safe to use as long as
+	 * 1) mmu_notifier_retry was checked after taking mmu_lock, and
+	 * 2) mmu_lock is taken now.
+	 *
+	 * We still need to disable IRQs to prevent concurrent tear down
+	 * of page tables.
+	 */
+	local_irq_save(flags);
+
+	pgd = READ_ONCE(*pgd_offset(kvm->mm, hva));
+	if (pgd_none(pgd))
+		goto out;
+
+	p4d = READ_ONCE(*p4d_offset(&pgd, hva));
+	if (p4d_none(p4d) || !p4d_present(p4d))
+		goto out;
 
+	pud = READ_ONCE(*pud_offset(&p4d, hva));
+	if (pud_none(pud) || !pud_present(pud))
+		goto out;
+
+	if (pud_large(pud)) {
+		level = PG_LEVEL_1G;
+		goto out;
+	}
+
+	pmd = READ_ONCE(*pmd_offset(&pud, hva));
+	if (pmd_none(pmd) || !pmd_present(pmd))
+		goto out;
+
+	if (pmd_large(pmd))
+		level = PG_LEVEL_2M;
+
+out:
+	local_irq_restore(flags);
 	return level;
 }