When doing nested virtualization, we may be able to read BNDCFGS but
still not be allowed to write to GUEST_BNDCFGS in the VMCS. Guard
writes to the field with vmx_mpx_supported(), and similarly hide the
MSR from userspace if the processor does not support the field.
We could work around this with the generic MSR save/load machinery,
but there is only a limited number of MSR save/load slots and it is
not really worthwhile to waste one for a scenario that should not
happen except in the nested virtualization case.
Based on a patch from Liu Jinsong, who reported it can also fix
problems with buggy VMX microcode.
Reported-by: Robert Hu <robert.hu@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/cpuid.c | 5 ++---
arch/x86/kvm/svm.c | 6 ++++++
arch/x86/kvm/vmx.c | 5 +++++
arch/x86/kvm/x86.c | 17 +++++++++++++++++
4 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 18aefb4d0927..64fae65730f3 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -47,7 +47,7 @@ u64 kvm_supported_xcr0(void)
{
u64 xcr0 = KVM_SUPPORTED_XCR0 & host_xcr0;
- if (!kvm_x86_ops->mpx_supported || !kvm_x86_ops->mpx_supported())
+ if (!kvm_x86_ops->mpx_supported())
xcr0 &= ~(XSTATE_BNDREGS | XSTATE_BNDCSR);
return xcr0;
@@ -259,8 +259,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
#endif
unsigned f_rdtscp = kvm_x86_ops->rdtscp_supported() ? F(RDTSCP) : 0;
unsigned f_invpcid = kvm_x86_ops->invpcid_supported() ? F(INVPCID) : 0;
- unsigned f_mpx = kvm_x86_ops->mpx_supported ?
- (kvm_x86_ops->mpx_supported() ? F(MPX) : 0) : 0;
+ unsigned f_mpx = kvm_x86_ops->mpx_supported() ? F(MPX) : 0;
/* cpuid 1.edx */
const u32 kvm_supported_word0_x86_features =
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index a449c3d76cba..2136cb6ab132 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4089,6 +4089,11 @@ static bool svm_invpcid_supported(void)
return false;
}
+static bool svm_mpx_supported(void)
+{
+ return false;
+}
+
static bool svm_has_wbinvd_exit(void)
{
return true;
@@ -4371,6 +4376,7 @@ static struct kvm_x86_ops svm_x86_ops = {
.rdtscp_supported = svm_rdtscp_supported,
.invpcid_supported = svm_invpcid_supported,
+ .mpx_supported = svm_mpx_supported,
.set_supported_cpuid = svm_set_supported_cpuid,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c95bea17fc1e..1320e0f8e611 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -729,6 +729,7 @@ static unsigned long nested_ept_get_cr3(struct kvm_vcpu *vcpu);
static u64 construct_eptp(unsigned long root_hpa);
static void kvm_cpu_vmxon(u64 addr);
static void kvm_cpu_vmxoff(void);
+static bool vmx_mpx_supported(void);
static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr);
static void vmx_set_segment(struct kvm_vcpu *vcpu,
struct kvm_segment *var, int seg);
@@ -2501,6 +2502,8 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
data = vmcs_readl(GUEST_SYSENTER_ESP);
break;
case MSR_IA32_BNDCFGS:
+ if (!vmx_mpx_supported())
+ return 1;
data = vmcs_read64(GUEST_BNDCFGS);
break;
case MSR_IA32_FEATURE_CONTROL:
@@ -2572,6 +2575,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
vmcs_writel(GUEST_SYSENTER_ESP, data);
break;
case MSR_IA32_BNDCFGS:
+ if (!vmx_mpx_supported())
+ return 1;
vmcs_write64(GUEST_BNDCFGS, data);
break;
case MSR_IA32_TSC:
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3f5fb4535f9c..aa986959f237 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3937,6 +3937,23 @@ static void kvm_init_msr_list(void)
for (i = j = KVM_SAVE_MSRS_BEGIN; i < ARRAY_SIZE(msrs_to_save); i++) {
if (rdmsr_safe(msrs_to_save[i], &dummy[0], &dummy[1]) < 0)
continue;
+
+ /*
+ * Even MSRs that are valid in the host may not be exposed
+ * to the guests in some cases. We could work around this
+ * in VMX with the generic MSR save/load machinery, but it
+ * is not really worthwhile since it will really only
+ * happen with nested virtualization.
+ */
+ switch (msrs_to_save[i]) {
+ case MSR_IA32_BNDCFGS:
+ if (!kvm_x86_ops->mpx_supported())
+ continue;
+ break;
+ default:
+ break;
+ }
+
if (j < i)
msrs_to_save[j] = msrs_to_save[i];
j++;
--
1.8.3.1
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