KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

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 ]---