[PATCH v2 4/4] KVM: x86/mmu: Fix a *very* theoretical race in kvm_mmu_track_write()

[Sean Christopherson]

Add full memory barriers in kvm_mmu_track_write() and account_shadowed()
to plug a (very, very theoretical) race where kvm_mmu_track_write() could
miss a 0->1 transition of indirect_shadow_pages and fail to zap relevant,
*stale* SPTEs.

Without the barriers, because modern x86 CPUs allow (per the SDM):

  Reads may be reordered with older writes to different locations but not
  with older writes to the same location.

it's (again, super theoretically) possible that the following could happen
(terms of values being visible/resolved):

 CPU0                          CPU1
 read memory[gfn] (=Y)
                               memory[gfn] Y=>X
                               read indirect_shadow_pages (=0)
 indirect_shadow_pages 0=>1

or conversely:

 CPU0                          CPU1
 indirect_shadow_pages 0=>1
                               read indirect_shadow_pages (=0)
 read memory[gfn] (=Y)
                               memory[gfn] Y=>X

In practice, this bug is likely benign as both the 0=>1 transition and
reordering of this scope are extremely rare occurrences.

Note, if the cost of the barrier (which is simply a locked ADD, see commit
450cbdd0125c ("locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE")),
is problematic, KVM could avoid the barrier by bailing earlier if checking
kvm_memslots_have_rmaps() is false.  But the odds of the barrier being
problematic is extremely low, *and* the odds of the extra checks being
meaningfully faster overall is also low.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 3c193b096b45..86b85060534d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -830,6 +830,14 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 	struct kvm_memory_slot *slot;
 	gfn_t gfn;
 
+	/*
+	 * Ensure indirect_shadow_pages is elevated prior to re-reading guest
+	 * child PTEs in FNAME(gpte_changed), i.e. guarantee either in-flight
+	 * emulated writes are visible before re-reading guest PTEs, or that
+	 * an emulated write will see the elevated count and acquire mmu_lock
+	 * to update SPTEs.  Pairs with the smp_mb() in kvm_mmu_track_write().
+	 */
+	smp_mb();
 	kvm->arch.indirect_shadow_pages++;
 	gfn = sp->gfn;
 	slots = kvm_memslots_for_spte_role(kvm, sp->role);
@@ -5747,10 +5755,15 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
 	bool flush = false;
 
 	/*
-	 * If we don't have indirect shadow pages, it means no page is
-	 * write-protected, so we can exit simply.
+	 * When emulating guest writes, ensure the written value is visible to
+	 * any task that is handling page faults before checking whether or not
+	 * KVM is shadowing a guest PTE.  This ensures either KVM will create
+	 * the correct SPTE in the page fault handler, or this task will see
+	 * a non-zero indirect_shadow_pages.  Pairs with the smp_mb() in
+	 * account_shadowed().
 	 */
-	if (!READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
+	smp_mb();
+	if (!vcpu->kvm->arch.indirect_shadow_pages)
 		return;
 
 	write_lock(&vcpu->kvm->mmu_lock);
-- 
2.43.0.594.gd9cf4e227d-goog