Commit 15d45071 authored by Ashok Raj's avatar Ashok Raj Committed by Thomas Gleixner
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KVM/x86: Add IBPB support

The Indirect Branch Predictor Barrier (IBPB) is an indirect branch
control mechanism. It keeps earlier branches from influencing
later ones.

Unlike IBRS and STIBP, IBPB does not define a new mode of operation.
It's a command that ensures predicted branch targets aren't used after
the barrier. Although IBRS and IBPB are enumerated by the same CPUID
enumeration, IBPB is very different.

IBPB helps mitigate against three potential attacks:

* Mitigate guests from being attacked by other guests.
  - This is addressed by issing IBPB when we do a guest switch.

* Mitigate attacks from guest/ring3->host/ring3.
  These would require a IBPB during context switch in host, or after
  VMEXIT. The host process has two ways to mitigate
  - Either it can be compiled with retpoline
  - If its going through context switch, and has set !dumpable then
    there is a IBPB in that path.
    (Tim's patch:
  - The case where after a VMEXIT you return back to Qemu might make
    Qemu attackable from guest when Qemu isn't compiled with retpoline.
  There are issues reported when doing IBPB on every VMEXIT that resulted
  in some tsc calibration woes in guest.

* Mitigate guest/ring0->host/ring0 attacks.
  When host kernel is using retpoline it is safe against these attacks.
  If host kernel isn't using retpoline we might need to do a IBPB flush on
  every VMEXIT.

Even when using retpoline for indirect calls, in certain conditions 'ret'
can use the BTB on Skylake-era CPUs. There are other mitigations
available like RSB stuffing/clearing.

* IBPB is issued only for SVM during svm_free_vcpu().
  VMX has a vmclear and SVM doesn't.  Follow discussion here:

Please refer to the following spec for more details on the enumeration
and control.

Refer here to get documentation about mitigations.

[peterz: rebase and changelog rewrite]
[karahmed: - rebase
           - vmx: expose PRED_CMD if guest has it in CPUID
           - svm: only pass through IBPB if guest has it in CPUID
           - vmx: support !cpu_has_vmx_msr_bitmap()]
           - vmx: support nested]
[dwmw2: Expose CPUID bit too (AMD IBPB only for now as we lack IBRS)
        PRED_CMD is a write-only MSR]

Signed-off-by: default avatarAshok Raj <>
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Signed-off-by: default avatarDavid Woodhouse <>
Signed-off-by: default avatarKarimAllah Ahmed <>
Signed-off-by: default avatarThomas Gleixner <>
Reviewed-by: default avatarKonrad Rzeszutek Wilk <>
Cc: Andrea Arcangeli <>
Cc: Andi Kleen <>
Cc: Asit Mallick <>
Cc: Linus Torvalds <>
Cc: Andy Lutomirski <>
Cc: Dave Hansen <>
Cc: Arjan Van De Ven <>
Cc: Greg KH <>
Cc: Jun Nakajima <>
Cc: Paolo Bonzini <>
Cc: Dan Williams <>
Cc: Tim Chen <>
parent b7b27aa0
......@@ -365,6 +365,10 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
F(3DNOWPREFETCH) | F(OSVW) | 0 /* IBS */ | F(XOP) |
0 /* SKINIT, WDT, LWP */ | F(FMA4) | F(TBM);
/* cpuid 0x80000008.ebx */
const u32 kvm_cpuid_8000_0008_ebx_x86_features =
/* cpuid 0xC0000001.edx */
const u32 kvm_cpuid_C000_0001_edx_x86_features =
......@@ -625,7 +629,12 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
if (!g_phys_as)
g_phys_as = phys_as;
entry->eax = g_phys_as | (virt_as << 8);
entry->ebx = entry->edx = 0;
entry->edx = 0;
/* IBPB isn't necessarily present in hardware cpuid */
if (boot_cpu_has(X86_FEATURE_IBPB))
entry->ebx |= F(IBPB);
entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
case 0x80000019:
......@@ -249,6 +249,7 @@ static const struct svm_direct_access_msrs {
{ .index = MSR_CSTAR, .always = true },
{ .index = MSR_SYSCALL_MASK, .always = true },
{ .index = MSR_IA32_PRED_CMD, .always = false },
{ .index = MSR_IA32_LASTBRANCHFROMIP, .always = false },
{ .index = MSR_IA32_LASTBRANCHTOIP, .always = false },
{ .index = MSR_IA32_LASTINTFROMIP, .always = false },
......@@ -529,6 +530,7 @@ struct svm_cpu_data {
struct kvm_ldttss_desc *tss_desc;
struct page *save_area;
struct vmcb *current_vmcb;
static DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
......@@ -1703,11 +1705,17 @@ static void svm_free_vcpu(struct kvm_vcpu *vcpu)
__free_pages(virt_to_page(svm->nested.msrpm), MSRPM_ALLOC_ORDER);
kmem_cache_free(kvm_vcpu_cache, svm);
* The vmcb page can be recycled, causing a false negative in
* svm_vcpu_load(). So do a full IBPB now.
static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
struct vcpu_svm *svm = to_svm(vcpu);
struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
int i;
if (unlikely(cpu != vcpu->cpu)) {
......@@ -1736,6 +1744,10 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
if (static_cpu_has(X86_FEATURE_RDTSCP))
wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
if (sd->current_vmcb != svm->vmcb) {
sd->current_vmcb = svm->vmcb;
avic_vcpu_load(vcpu, cpu);
......@@ -3684,6 +3696,22 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr);
if (!msr->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
return 1;
if (data & ~PRED_CMD_IBPB)
return 1;
if (!data)
if (is_guest_mode(vcpu))
set_msr_interception(svm->msrpm, MSR_IA32_PRED_CMD, 0, 1);
case MSR_STAR:
svm->vmcb-> = data;
......@@ -593,6 +593,7 @@ struct vcpu_vmx {
u64 msr_host_kernel_gs_base;
u64 msr_guest_kernel_gs_base;
u32 vm_entry_controls_shadow;
u32 vm_exit_controls_shadow;
u32 secondary_exec_control;
......@@ -934,6 +935,8 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked);
static bool nested_vmx_is_page_fault_vmexit(struct vmcs12 *vmcs12,
u16 error_code);
static void vmx_update_msr_bitmap(struct kvm_vcpu *vcpu);
static void __always_inline vmx_disable_intercept_for_msr(unsigned long *msr_bitmap,
u32 msr, int type);
static DEFINE_PER_CPU(struct vmcs *, vmxarea);
static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
......@@ -1905,6 +1908,29 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
vmcs_write32(EXCEPTION_BITMAP, eb);
* Check if MSR is intercepted for L01 MSR bitmap.
static bool msr_write_intercepted_l01(struct kvm_vcpu *vcpu, u32 msr)
unsigned long *msr_bitmap;
int f = sizeof(unsigned long);
if (!cpu_has_vmx_msr_bitmap())
return true;
msr_bitmap = to_vmx(vcpu)->vmcs01.msr_bitmap;
if (msr <= 0x1fff) {
return !!test_bit(msr, msr_bitmap + 0x800 / f);
} else if ((msr >= 0xc0000000) && (msr <= 0xc0001fff)) {
msr &= 0x1fff;
return !!test_bit(msr, msr_bitmap + 0xc00 / f);
return true;
static void clear_atomic_switch_msr_special(struct vcpu_vmx *vmx,
unsigned long entry, unsigned long exit)
......@@ -2283,6 +2309,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
if (!already_loaded) {
......@@ -3340,6 +3367,34 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr_info);
if (!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_IBPB) &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
return 1;
if (data & ~PRED_CMD_IBPB)
return 1;
if (!data)
* For non-nested:
* When it's written (to non-zero) for the first time, pass
* it through.
* For nested:
* The handling of the MSR bitmap for L2 guests is done in
* nested_vmx_merge_msr_bitmap. We should not touch the
* vmcs02.msr_bitmap here since it gets completely overwritten
* in the merging.
vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
case MSR_IA32_CR_PAT:
if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
......@@ -10042,9 +10097,23 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
struct page *page;
unsigned long *msr_bitmap_l1;
unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.vmcs02.msr_bitmap;
* pred_cmd is trying to verify two things:
* 1. L0 gave a permission to L1 to actually passthrough the MSR. This
* ensures that we do not accidentally generate an L02 MSR bitmap
* from the L12 MSR bitmap that is too permissive.
* 2. That L1 or L2s have actually used the MSR. This avoids
* unnecessarily merging of the bitmap if the MSR is unused. This
* works properly because we only update the L01 MSR bitmap lazily.
* So even if L0 should pass L1 these MSRs, the L01 bitmap is only
* updated to reflect this when L1 (or its L2s) actually write to
* the MSR.
bool pred_cmd = msr_write_intercepted_l01(vcpu, MSR_IA32_PRED_CMD);
/* This shortcut is ok because we support only x2APIC MSRs so far. */
if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
if (!nested_cpu_has_virt_x2apic_mode(vmcs12) &&
return false;
page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->msr_bitmap);
......@@ -10077,6 +10146,13 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
if (pred_cmd)
msr_bitmap_l1, msr_bitmap_l0,
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