| Message ID | 20240716022014.240960-2-mlevitsk@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Fix for a very old KVM bug in the segment cache | expand |
On Mon, Jul 15, 2024, Maxim Levitsky wrote: > In prepare_vmcs02_rare, call vmx_segment_cache_clear, instead > of setting the segment_cache.bitmask directly. > > No functional change intended. > > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com> > --- > arch/x86/kvm/vmx/nested.c | 5 +++-- > arch/x86/kvm/vmx/vmx.c | 4 ---- > arch/x86/kvm/vmx/vmx.h | 5 +++++ > 3 files changed, 8 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index 643935a0f70ab..d3ca1a772ae67 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -2469,6 +2469,9 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) > > if (!hv_evmcs || !(hv_evmcs->hv_clean_fields & > HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) { > + > + vmx_segment_cache_clear(vmx); > + > vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector); > vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector); > vmcs_write16(GUEST_SS_SELECTOR, vmcs12->guest_ss_selector); > @@ -2505,8 +2508,6 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) > vmcs_writel(GUEST_TR_BASE, vmcs12->guest_tr_base); > vmcs_writel(GUEST_GDTR_BASE, vmcs12->guest_gdtr_base); > vmcs_writel(GUEST_IDTR_BASE, vmcs12->guest_idtr_base); > - > - vmx->segment_cache.bitmask = 0; This actually exacerbates the bug that you're trying fix in patch 2. Clearing segment_cache.bitmask _after_ writing the relevant state limits the stale data to only the accessor that's running in IRQ context (kvm_arch_vcpu_put()). Clearing segment_cache.bitmask _before_ writing the relevant statement means that kvm_arch_vcpu_put() _and_ all future readers will be exposed to the stale data, because the stale data cached by kvm_arch_vcpu_put() won't mark it invalid.
On Tue, 2024-07-16 at 14:07 -0700, Sean Christopherson wrote: > On Mon, Jul 15, 2024, Maxim Levitsky wrote: > > In prepare_vmcs02_rare, call vmx_segment_cache_clear, instead > > of setting the segment_cache.bitmask directly. > > > > No functional change intended. > > > > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com> > > --- > > arch/x86/kvm/vmx/nested.c | 5 +++-- > > arch/x86/kvm/vmx/vmx.c | 4 ---- > > arch/x86/kvm/vmx/vmx.h | 5 +++++ > > 3 files changed, 8 insertions(+), 6 deletions(-) > > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > > index 643935a0f70ab..d3ca1a772ae67 100644 > > --- a/arch/x86/kvm/vmx/nested.c > > +++ b/arch/x86/kvm/vmx/nested.c > > @@ -2469,6 +2469,9 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) > > > > if (!hv_evmcs || !(hv_evmcs->hv_clean_fields & > > HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) { > > + > > + vmx_segment_cache_clear(vmx); > > + > > vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector); > > vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector); > > vmcs_write16(GUEST_SS_SELECTOR, vmcs12->guest_ss_selector); > > @@ -2505,8 +2508,6 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) > > vmcs_writel(GUEST_TR_BASE, vmcs12->guest_tr_base); > > vmcs_writel(GUEST_GDTR_BASE, vmcs12->guest_gdtr_base); > > vmcs_writel(GUEST_IDTR_BASE, vmcs12->guest_idtr_base); > > - > > - vmx->segment_cache.bitmask = 0; > > This actually exacerbates the bug that you're trying fix in patch 2. Clearing > segment_cache.bitmask _after_ writing the relevant state limits the stale data > to only the accessor that's running in IRQ context (kvm_arch_vcpu_put()). > > Clearing segment_cache.bitmask _before_ writing the relevant statement means > that kvm_arch_vcpu_put() _and_ all future readers will be exposed to the stale > data, because the stale data cached by kvm_arch_vcpu_put() won't mark it invalid. > I noticed that after I sent the patch series, this makes sense. Best regards, Maxim Levitsky
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 643935a0f70ab..d3ca1a772ae67 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -2469,6 +2469,9 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) if (!hv_evmcs || !(hv_evmcs->hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) { + + vmx_segment_cache_clear(vmx); + vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector); vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector); vmcs_write16(GUEST_SS_SELECTOR, vmcs12->guest_ss_selector); @@ -2505,8 +2508,6 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) vmcs_writel(GUEST_TR_BASE, vmcs12->guest_tr_base); vmcs_writel(GUEST_GDTR_BASE, vmcs12->guest_gdtr_base); vmcs_writel(GUEST_IDTR_BASE, vmcs12->guest_idtr_base); - - vmx->segment_cache.bitmask = 0; } if (!hv_evmcs || !(hv_evmcs->hv_clean_fields & diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index b3c83c06f8265..fa9f307d9b18b 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -524,10 +524,6 @@ static const struct kvm_vmx_segment_field { VMX_SEGMENT_FIELD(LDTR), }; -static inline void vmx_segment_cache_clear(struct vcpu_vmx *vmx) -{ - vmx->segment_cache.bitmask = 0; -} static unsigned long host_idt_base; diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h index 7b64e271a9319..1689f0d59f435 100644 --- a/arch/x86/kvm/vmx/vmx.h +++ b/arch/x86/kvm/vmx/vmx.h @@ -755,4 +755,9 @@ static inline bool vmx_can_use_ipiv(struct kvm_vcpu *vcpu) return lapic_in_kernel(vcpu) && enable_ipiv; } +static inline void vmx_segment_cache_clear(struct vcpu_vmx *vmx) +{ + vmx->segment_cache.bitmask = 0; +} + #endif /* __KVM_X86_VMX_H */
In prepare_vmcs02_rare, call vmx_segment_cache_clear, instead of setting the segment_cache.bitmask directly. No functional change intended. Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com> --- arch/x86/kvm/vmx/nested.c | 5 +++-- arch/x86/kvm/vmx/vmx.c | 4 ---- arch/x86/kvm/vmx/vmx.h | 5 +++++ 3 files changed, 8 insertions(+), 6 deletions(-)