KVM: x86: fix L1TF's MMIO GFN calculation

One defense against L1TF in KVM is to always set the upper five bits
of the *legal* physical address in the SPTEs for non-present and
reserved SPTEs, e.g. MMIO SPTEs.  In the MMIO case, the GFN of the
MMIO SPTE may overlap with the upper five bits that are being usurped
to defend against L1TF.  To preserve the GFN, the bits of the GFN that
overlap with the repurposed bits are shifted left into the reserved
bits, i.e. the GFN in the SPTE will be split into high and low parts.
When retrieving the GFN from the MMIO SPTE, e.g. to check for an MMIO
access, get_mmio_spte_gfn() unshifts the affected bits and restores
the original GFN for comparison.  Unfortunately, get_mmio_spte_gfn()
neglects to mask off the reserved bits in the SPTE that were used to
store the upper chunk of the GFN.  As a result, KVM fails to detect
MMIO accesses whose GPA overlaps the repurprosed bits, which in turn
causes guest panics and hangs.

Fix the bug by generating a mask that covers the lower chunk of the
GFN, i.e. the bits that aren't shifted by the L1TF mitigation.  The
alternative approach would be to explicitly zero the five reserved
bits that are used to store the upper chunk of the GFN, but that
requires additional run-time computation and makes an already-ugly
bit of code even more inscrutable.

I considered adding a WARN_ON_ONCE(low_phys_bits-1 <= PAGE_SHIFT) to
warn if GENMASK_ULL() generated a nonsensical value, but that seemed
silly since that would mean a system that supports VMX has less than
18 bits of physical address space...

Reported-by: Sakari Ailus <sakari.ailus@iki.fi>
Fixes: d9b47449c1a1 ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
Cc: Junaid Shahid <junaids@google.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: stable@vger.kernel.org
Reviewed-by: Junaid Shahid <junaids@google.com>
Tested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -249,6 +249,17 @@ static u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
  */
 static const u64 shadow_nonpresent_or_rsvd_mask_len = 5;
 
+/*
+ * In some cases, we need to preserve the GFN of a non-present or reserved
+ * SPTE when we usurp the upper five bits of the physical address space to
+ * defend against L1TF, e.g. for MMIO SPTEs.  To preserve the GFN, we'll
+ * shift bits of the GFN that overlap with shadow_nonpresent_or_rsvd_mask
+ * left into the reserved bits, i.e. the GFN in the SPTE will be split into
+ * high and low parts.  This mask covers the lower bits of the GFN.
+ */
+static u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
+
+
 static void mmu_spte_set(u64 *sptep, u64 spte);
 static union kvm_mmu_page_role
 kvm_mmu_calc_root_page_role(struct kvm_vcpu *vcpu);
@@ -357,9 +368,7 @@ static bool is_mmio_spte(u64 spte)
 
 static gfn_t get_mmio_spte_gfn(u64 spte)
 {
-	u64 mask = generation_mmio_spte_mask(MMIO_GEN_MASK) | shadow_mmio_mask |
-		   shadow_nonpresent_or_rsvd_mask;
-	u64 gpa = spte & ~mask;
+	u64 gpa = spte & shadow_nonpresent_or_rsvd_lower_gfn_mask;
 
 	gpa |= (spte >> shadow_nonpresent_or_rsvd_mask_len)
 	       & shadow_nonpresent_or_rsvd_mask;
@@ -423,6 +432,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_set_mask_ptes);
 
 static void kvm_mmu_reset_all_pte_masks(void)
 {
+	u8 low_phys_bits;
+
 	shadow_user_mask = 0;
 	shadow_accessed_mask = 0;
 	shadow_dirty_mask = 0;
@@ -437,12 +448,17 @@ static void kvm_mmu_reset_all_pte_masks(void)
 	 * appropriate mask to guard against L1TF attacks. Otherwise, it is
 	 * assumed that the CPU is not vulnerable to L1TF.
 	 */
+	low_phys_bits = boot_cpu_data.x86_phys_bits;
 	if (boot_cpu_data.x86_phys_bits <
-	    52 - shadow_nonpresent_or_rsvd_mask_len)
+	    52 - shadow_nonpresent_or_rsvd_mask_len) {
 		shadow_nonpresent_or_rsvd_mask =
 			rsvd_bits(boot_cpu_data.x86_phys_bits -
 				  shadow_nonpresent_or_rsvd_mask_len,
 				  boot_cpu_data.x86_phys_bits - 1);
+		low_phys_bits -= shadow_nonpresent_or_rsvd_mask_len;
+	}
+	shadow_nonpresent_or_rsvd_lower_gfn_mask =
+		GENMASK_ULL(low_phys_bits - 1, PAGE_SHIFT);
 }
 
 static int is_cpuid_PSE36(void)