[PATCH 6.1 275/312] KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

Greg Kroah-Hartman
2026-04-08T18:18:39Z

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit aad885e774966e97b675dfe928da164214a71605 ]

When installing an emulated MMIO SPTE, do so *after* dropping/zapping the
existing SPTE (if it's shadow-present). While commit a54aa15c6bda3 was
right about it being impossible to convert a shadow-present SPTE to an
MMIO SPTE due to a _guest_ write, it failed to account for writes to guest
memory that are outside the scope of KVM.

E.g. if host userspace modifies a shadowed gPTE to switch from a memslot
to emulted MMIO and then the guest hits a relevant page fault, KVM will
install the MMIO SPTE without first zapping the shadow-present SPTE.

------------[ cut here ]------------
is_shadow_present_pte(*sptep)
WARNING: arch/x86/kvm/mmu/mmu.c:484 at mark_mmio_spte+0xb2/0xc0 [kvm], CPU#0: vmx_ept_stale_r/4292
Modules linked in: kvm_intel kvm irqbypass
CPU: 0 UID: 1000 PID: 4292 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:mark_mmio_spte+0xb2/0xc0 [kvm]
Call Trace:
<TASK>
mmu_set_spte+0x237/0x440 [kvm]
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
RIP: 0033:0x47fa3f
</TASK>
---[ end trace 0000000000000000 ]---

Reported-by: Alexander Bulekov <bkov@amazon.com>
Debugged-by: Alexander Bulekov <bkov@amazon.com>
Suggested-by: Fred Griffoul <fgriffo@amazon.co.uk>
Fixes: a54aa15c6bda3 ("KVM: x86/mmu: Handle MMIO SPTEs directly in mmu_set_spte()")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
[ replaced kvm_flush_remote_tlbs_gfn() with kvm_flush_remote_tlbs_with_address() and preserved pgprintk call ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/mmu/mmu.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2814,12 +2814,6 @@ static int mmu_set_spte(struct kvm_vcpu
pgprintk("%s: spte %llx write_fault %d gfn %llx\n", __func__,
*sptep, write_fault, gfn);

- if (unlikely(is_noslot_pfn(pfn))) {
- vcpu->stat.pf_mmio_spte_created++;
- mark_mmio_spte(vcpu, sptep, gfn, pte_access);
- return RET_PF_EMULATE;
- }
-
if (is_shadow_present_pte(*sptep)) {
/*
* If we overwrite a PTE page pointer with a 2MB PMD, unlink
@@ -2841,6 +2835,15 @@ static int mmu_set_spte(struct kvm_vcpu
was_rmapped = 1;
}

+ if (unlikely(is_noslot_pfn(pfn))) {
+ vcpu->stat.pf_mmio_spte_created++;
+ mark_mmio_spte(vcpu, sptep, gfn, pte_access);
+ if (flush)
+ kvm_flush_remote_tlbs_with_address(vcpu->kvm, gfn,
+ KVM_PAGES_PER_HPAGE(level));
+ return RET_PF_EMULATE;
+ }
+
wrprot = make_spte(vcpu, sp, slot, pte_access, gfn, pfn, *sptep, prefetch,
true, host_writable, &spte);