KVM: x86/mmu: Only WARN in direct MMUs when overwriting shadow-present SPTE

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

KVM: x86/mmu: Only WARN in direct MMUs when overwriting shadow-present SPTE

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 ]---