[PATCH v4 1/9] KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge page

Sean Christopherson
2022-08-30T23:55:50Z

WARN and kill the VM if KVM attempts to double count an NX huge page,
i.e. attempts to re-tag a shadow page with "NX huge page disallowed".
KVM does NX huge page accounting only when linking a new shadow page, and
it should be impossible for a new shadow page to be already accounted.
E.g. even in the TDP MMU case, where vCPUs can race to install a new
shadow page, only the "winner" will account the installed page.

Kill the VM instead of continuing on as either KVM has an egregious bug,
e.g. didn't zero-initialize the data, or there's host data corruption, in
which carrying on is dangerous, e.g. could cause silent data corruption
in the guest.

Reported-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
---
arch/x86/kvm/mmu/mmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 32b60a6b83bd..74afee3f2476 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -804,7 +804,7 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)

void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp)
{
- if (sp->lpage_disallowed)
+ if (KVM_BUG_ON(sp->lpage_disallowed, kvm))
return;

++kvm->stat.nx_lpage_splits;
--
2.37.2.672.g94769d06f0-goog