Re: [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

Sean Christopherson
2023-03-30T20:33:12Z

On Thu, Mar 30, 2023, Sean Christopherson wrote:
> On Thu, Mar 30, 2023, Mathias Krause wrote:
> > On 22.03.23 02:37, Mathias Krause wrote:
> > I'm leaning to make CR0.WP guest owned only iff we're running on bare
> > metal or the VMM is KVM to not play whack-a-mole for all the VMMs that
> > might have similar bugs. (Will try to test a few others here as well.)
> > However, that would prevent them from getting the performance gain, so
> > I'd rather have this fixed / worked around in KVM instead.
>
> Before we start putting bandaids on this, let's (a) confirm this appears to be
> an issue with ESXi and (b) pull in VMware folks to get their input.
>
> > Any ideas how to investigate this further?
>
> Does the host in question support UMIP?
>
> Can you capture a tracepoint log from L1 to verify that L1 KVM is _not_ injecting
> any kind of exception? E.g. to get the KVM kitchen sink:
>
> echo 1 > /sys/kernel/debug/tracing/tracing_on
> echo 1 > /sys/kernel/debug/tracing/events/kvm/enable
>
> cat /sys/kernel/debug/tracing/trace > log
>
> Or if that's too noisy, a more targeted trace (exception injection + emulation)
> woud be:
>
> echo 1 > /sys/kernel/debug/tracing/tracing_on
>
> echo 1 > /sys/kernel/debug/tracing/events/kvm/kvm_emulate_insn/enable
> echo 1 > /sys/kernel/debug/tracing/events/kvm/kvm_inj_exception/enable
> echo 1 > /sys/kernel/debug/tracing/events/kvm/kvm_entry/enable
> echo 1 > /sys/kernel/debug/tracing/events/kvm/kvm_exit/enable

Duh, this is likely a KVM bug. I expect the issue will go away if you revert

fb509f76acc8 ("KVM: VMX: Make CR0.WP a guest owned bit")

KVM doesn't consume CR0.WP for _its_ MMU, but it does consume CR0.WP for the
guest walker. By passing through CR0.WP, toggling only CR0.WP will not trap
(obviously) and thus won't run through kvm_post_set_cr0(), thus resulting in stle
information due to not invoking kvm_init_mmu().

I'm preetty sure I even called that we needed to refresh the permissions, but then
obviously forgot to actually make that happen.

I believe this will remedy the issue. If it does, I'll post a proper patch
(likely won't be until next week). Compile tested only.

---
arch/x86/kvm/mmu.h | 8 +++++++-
arch/x86/kvm/mmu/mmu.c | 14 ++++++++++++++
2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 89f532516a45..4a303aa735dd 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -113,6 +113,7 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu);
int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
u64 fault_address, char *insn, int insn_len);
+void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu);

int kvm_mmu_load(struct kvm_vcpu *vcpu);
void kvm_mmu_unload(struct kvm_vcpu *vcpu);
@@ -184,8 +185,13 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
u64 implicit_access = access & PFERR_IMPLICIT_ACCESS;
bool not_smap = ((rflags & X86_EFLAGS_AC) | implicit_access) == X86_EFLAGS_AC;
int index = (pfec + (not_smap << PFERR_RSVD_BIT)) >> 1;
- bool fault = (mmu->permissions[index] >> pte_access) & 1;
u32 errcode = PFERR_PRESENT_MASK;
+ bool fault;
+
+ if (tdp_enabled)
+ kvm_mmu_refresh_passthrough_bits(vcpu, mmu);
+
+ fault = (mmu->permissions[index] >> pte_access) & 1;

WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
if (unlikely(mmu->pkru_mask)) {
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4c874d4ec68f..2a63b5725f36 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5186,6 +5186,20 @@ static union kvm_cpu_role kvm_calc_cpu_role(struct kvm_vcpu *vcpu,
return role;
}

+void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
+{
+ const bool cr0_wp = kvm_is_cr0_bit_set(vcpu, X86_CR0_WP);
+
+ BUILD_BUG_ON((KVM_MMU_CR0_ROLE_BITS & KVM_POSSIBLE_CR0_GUEST_BITS) != X86_CR0_WP);
+ BUILD_BUG_ON((KVM_MMU_CR4_ROLE_BITS & KVM_POSSIBLE_CR4_GUEST_BITS));
+
+ if (is_cr0_wp(mmu) == cr0_wp)
+ return;
+
+ mmu->cpu_role.base.cr0_wp = cr0_wp;
+ reset_guest_paging_metadata(vcpu, mmu);
+}
+
static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
{
/* tdp_root_level is architecture forced level, use it if nonzero */

base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393
--