[PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation

[Paolo Bonzini]

The SPTE_MMIO_MASK overlaps with the bits used to track MMIO
generation number.  A high enough generation number would overwrite the
SPTE_SPECIAL_MASK region and cause the MMIO SPTE to be misinterpreted;
likewise, setting bits 52 and 53 would also cause an incorrect generation
number to be read from the PTE.

Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
Reported-by: Ben Gardon <bgardon@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 57e4dbddba72..e34ca43d9166 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -418,22 +418,25 @@ static inline bool is_access_track_spte(u64 spte)
  * requires a full MMU zap).  The flag is instead explicitly queried when
  * checking for MMIO spte cache hits.
  */
-#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(18, 0)
+#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(17, 0)
 
 #define MMIO_SPTE_GEN_LOW_START		3
 #define MMIO_SPTE_GEN_LOW_END		11
 #define MMIO_SPTE_GEN_LOW_MASK		GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
 						    MMIO_SPTE_GEN_LOW_START)
 
-#define MMIO_SPTE_GEN_HIGH_START	52
-#define MMIO_SPTE_GEN_HIGH_END		61
+/* Leave room for SPTE_SPECIAL_MASK.  */
+#define MMIO_SPTE_GEN_HIGH_START	54
+#define MMIO_SPTE_GEN_HIGH_END		62
 #define MMIO_SPTE_GEN_HIGH_MASK		GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
 						    MMIO_SPTE_GEN_HIGH_START)
+
 static u64 generation_mmio_spte_mask(u64 gen)
 {
 	u64 mask;
 
 	WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
+	BUILD_BUG_ON(MMIO_SPTE_GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT);
 
 	mask = (gen << MMIO_SPTE_GEN_LOW_START) & MMIO_SPTE_GEN_LOW_MASK;
 	mask |= (gen << MMIO_SPTE_GEN_HIGH_START) & MMIO_SPTE_GEN_HIGH_MASK;
-- 
1.8.3.1