[PATCH v2 2/4] KVM: x86/mmu: Exclude the MMU_PRESENT bit from MMIO SPTE's generation

[Sean Christopherson <seanjc@google.com>]

Drop bit 11, used for the MMU_PRESENT flag, from the set of bits used to
store the generation number in MMIO SPTEs.  MMIO SPTEs with bit 11 set,
which occurs when userspace creates 128+ memslots in an address space,
get false positives for is_shadow_present_spte(), which lead to a variety
of fireworks, crashes KVM, and likely hangs the host kernel.

Fixes: b14e28f37e9b ("KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/spte.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index b53036d9ddf3..bca0ba11cccf 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -101,11 +101,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
 #undef SHADOW_ACC_TRACK_SAVED_MASK
 
 /*
- * Due to limited space in PTEs, the MMIO generation is a 20 bit subset of
+ * Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
  * the memslots generation and is derived as follows:
  *
- * Bits 0-8 of the MMIO generation are propagated to spte bits 3-11
- * Bits 9-19 of the MMIO generation are propagated to spte bits 52-62
+ * Bits 0-7 of the MMIO generation are propagated to spte bits 3-10
+ * Bits 8-18 of the MMIO generation are propagated to spte bits 52-62
  *
  * The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
  * the MMIO generation number, as doing so would require stealing a bit from
@@ -116,7 +116,7 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
  */
 
 #define MMIO_SPTE_GEN_LOW_START		3
-#define MMIO_SPTE_GEN_LOW_END		11
+#define MMIO_SPTE_GEN_LOW_END		10
 
 #define MMIO_SPTE_GEN_HIGH_START	52
 #define MMIO_SPTE_GEN_HIGH_END		62
@@ -125,12 +125,14 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
 						    MMIO_SPTE_GEN_LOW_START)
 #define MMIO_SPTE_GEN_HIGH_MASK		GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
 						    MMIO_SPTE_GEN_HIGH_START)
+static_assert(!(SPTE_MMU_PRESENT_MASK &
+		(MMIO_SPTE_GEN_LOW_MASK | MMIO_SPTE_GEN_HIGH_MASK)));
 
 #define MMIO_SPTE_GEN_LOW_BITS		(MMIO_SPTE_GEN_LOW_END - MMIO_SPTE_GEN_LOW_START + 1)
 #define MMIO_SPTE_GEN_HIGH_BITS		(MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1)
 
 /* remember to adjust the comment above as well if you change these */
-static_assert(MMIO_SPTE_GEN_LOW_BITS == 9 && MMIO_SPTE_GEN_HIGH_BITS == 11);
+static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
 
 #define MMIO_SPTE_GEN_LOW_SHIFT		(MMIO_SPTE_GEN_LOW_START - 0)
 #define MMIO_SPTE_GEN_HIGH_SHIFT	(MMIO_SPTE_GEN_HIGH_START - MMIO_SPTE_GEN_LOW_BITS)
-- 
2.30.1.766.gb4fecdf3b7-goog