[PATCH 6.19 251/342] KVM: x86/mmu: Only WARN in direct MMUs when overwriting shadow-present SPTE

Greg Kroah-Hartman
2026-03-31T16:42:06Z

From: Sean Christopherson <seanjc@google.com>

commit df83746075778958954aa0460cca55f4b3fc9c02 upstream.

Adjust KVM's sanity check against overwriting a shadow-present SPTE with a
another SPTE with a different target PFN to only apply to direct MMUs,
i.e. only to MMUs without shadowed gPTEs. While it's impossible for KVM
to overwrite a shadow-present SPTE in response to a guest write, writes
from outside the scope of KVM, e.g. from host userspace, aren't detected
by KVM's write tracking and so can break KVM's shadow paging rules.

------------[ cut here ]------------
pfn != spte_to_pfn(*sptep)
WARNING: arch/x86/kvm/mmu/mmu.c:3069 at mmu_set_spte+0x1e4/0x440 [kvm], CPU#0: vmx_ept_stale_r/872
Modules linked in: kvm_intel kvm irqbypass
CPU: 0 UID: 1000 PID: 872 Comm: vmx_ept_stale_r Not tainted 7.0.0-rc2-eafebd2d2ab0-sink-vm #319 PREEMPT
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:mmu_set_spte+0x1e4/0x440 [kvm]
Call Trace:
<TASK>
ept_page_fault+0x535/0x7f0 [kvm]
kvm_mmu_do_page_fault+0xee/0x1f0 [kvm]
kvm_mmu_page_fault+0x8d/0x620 [kvm]
vmx_handle_exit+0x18c/0x5a0 [kvm_intel]
kvm_arch_vcpu_ioctl_run+0xc55/0x1c20 [kvm]
kvm_vcpu_ioctl+0x2d5/0x980 [kvm]
__x64_sys_ioctl+0x8a/0xd0
do_syscall_64+0xb5/0x730
entry_SYSCALL_64_after_hwframe+0x4b/0x53
</TASK>
---[ end trace 0000000000000000 ]---

Fixes: 11d45175111d ("KVM: x86/mmu: Warn if PFN changes on shadow-present SPTE in shadow MMU")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/mmu/mmu.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3060,7 +3060,8 @@ static int mmu_set_spte(struct kvm_vcpu
child = spte_to_child_sp(pte);
drop_parent_pte(vcpu->kvm, child, sptep);
flush = true;
- } else if (WARN_ON_ONCE(pfn != spte_to_pfn(*sptep))) {
+ } else if (pfn != spte_to_pfn(*sptep)) {
+ WARN_ON_ONCE(vcpu->arch.mmu->root_role.direct);
drop_spte(vcpu->kvm, sptep);
flush = true;
} else