[PATCH] KVM: x86: Clamp guest MAXPHYADDR to the architectural maximum

[Pablo Azuaje]

cpuid_query_maxphyaddr() returns CPUID.0x80000008.EAX[7:0] as the
guest's physical-address width without any sanity check.  KVM computes
the reserved GPA bits from that value, e.g.
kvm_vcpu_reserved_gpa_bits_raw() is rsvd_bits(maxphyaddr, 63), and
rsvd_bits() returns 0 when the bit range is empty.  A VMM that
populates CPUID.0x80000008.EAX above 52 therefore makes KVM stop
treating the upper bits of guest PTEs as reserved: reserved-bit fault
checking on guest page table walks silently degenerates, and garbage
nCR3s/PDPTRs are accepted instead of faulting.

A MAXPHYADDR above 52 is architecturally impossible on x86-64; clamp
the queried value to 52 so that a buggy VMM configuration degrades to a
(very wide) but still meaningful physical-address width instead of
disabling KVM's reserved-bit accounting.

This is hardening rather than a security fix: the actor is the VMM,
which already owns the guest, so an over-wide MAXPHYADDR is not a
privilege boundary crossing.  The motivation is correctness, not
containment: a configuration value outside architectural limits should
not silently change how KVM validates guest page tables, and clamping
keeps the degenerate case well-defined instead of subtly disabling a
whole class of fault checks.  The unclamped read predates the
introduction of the cached value; no Fixes: tag is attached for that
reason.

Signed-off-by: Pablo Azuaje <pablo45154@xxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 851f151efb35..f0166072968f 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -477,8 +477,18 @@ int cpuid_query_maxphyaddr(struct kvm_vcpu *vcpu)
 	if (!best || best->eax < 0x80000008)
 		goto not_found;
 	best = kvm_find_cpuid_entry(vcpu, 0x80000008);
-	if (best)
-		return best->eax & 0xff;
+	if (best) {
+		/*
+		 * Clamp MAXPHYADDR to the architectural maximum of 52 bits.
+		 * A larger value is a VMM bug: KVM derives the reserved GPA
+		 * bits from MAXPHYADDR, e.g. kvm_vcpu_reserved_gpa_bits_raw()
+		 * is rsvd_bits(maxphyaddr, 63), and rsvd_bits() returns 0
+		 * when the range is empty, i.e. an over-wide MAXPHYADDR
+		 * silently disables reserved-bit checks on guest page table
+		 * walks.
+		 */
+		return min_t(u32, best->eax & 0xff, 52);
+	}
 not_found:
 	return 36;
 }
-- 
2.53.0