[PATCH 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs

David Woodhouse
2018-02-14T13:44:58Z

The original IBRS hack in microcode is horribly slow. For the next
generation of CPUs, as a stopgap until we get a proper fix, Intel
promise an "Enhanced IBRS" which will be fast.

The assumption is that predictions in the BTB/RSB will be tagged with
the VMX mode and ring that they were learned in, and thus the CPU will
avoid consuming unsafe predictions without a performance penalty.

Intel's documentation says that it is still required to set the IBRS bit
in the SPEC_CTRL MSR and ensure that it remains set.

Cope with this by trapping and emulating *all* access to SPEC_CTRL from
KVM guests when the IBRS_ALL feature is present, so it can never be
turned off. Guests who see IBRS_ALL should never do anything except
turn it on at boot anyway. And if they didn't know about IBRS_ALL and
they keep frobbing IBRS on every kernel entry/exit... well the vmexit
for a no-op is probably going to be faster than they were expecting
anyway, so they'll live.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Arjan van de Ven <arjan.van.de.ven@intel.com>
---
arch/x86/include/asm/nospec-branch.h | 9 ++++++++-
arch/x86/kernel/cpu/bugs.c | 16 ++++++++++++++--
arch/x86/kvm/vmx.c | 31 +++++++++++++++++++------------
3 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 0995c6a..34cbce3 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -141,9 +141,16 @@ enum spectre_v2_mitigation {
SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
SPECTRE_V2_RETPOLINE_GENERIC,
SPECTRE_V2_RETPOLINE_AMD,
- SPECTRE_V2_IBRS,
+ SPECTRE_V2_IBRS_ALL,
};

+extern enum spectre_v2_mitigation spectre_v2_enabled;
+
+static inline bool spectre_v2_ibrs_all(void)
+{
+ return spectre_v2_enabled == SPECTRE_V2_IBRS_ALL;
+}
+
extern char __indirect_thunk_start[];
extern char __indirect_thunk_end[];

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bfca937..eefd900 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -88,12 +88,13 @@ static const char *spectre_v2_strings[] = {
[SPECTRE_V2_RETPOLINE_MINIMAL_AMD] = "Vulnerable: Minimal AMD ASM retpoline",
[SPECTRE_V2_RETPOLINE_GENERIC] = "Mitigation: Full generic retpoline",
[SPECTRE_V2_RETPOLINE_AMD] = "Mitigation: Full AMD retpoline",
+ [SPECTRE_V2_IBRS_ALL] = "Mitigation: Enhanced IBRS",
};

#undef pr_fmt
#define pr_fmt(fmt) "Spectre V2 : " fmt

-static enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;
+enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;

#ifdef RETPOLINE
static bool spectre_v2_bad_module;
@@ -237,6 +238,16 @@ static void __init spectre_v2_select_mitigation(void)

case SPECTRE_V2_CMD_FORCE:
case SPECTRE_V2_CMD_AUTO:
+ if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES)) {
+ u64 ia32_cap = 0;
+
+ rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
+ if (ia32_cap & ARCH_CAP_IBRS_ALL) {
+ mode = SPECTRE_V2_IBRS_ALL;
+ wrmsrl(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS);
+ goto ibrs_all;
+ }
+ }
if (IS_ENABLED(CONFIG_RETPOLINE))
goto retpoline_auto;
break;
@@ -274,6 +285,7 @@ static void __init spectre_v2_select_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
}

+ ibrs_all:
spectre_v2_enabled = mode;
pr_info("%s\n", spectre_v2_strings[mode]);

@@ -305,7 +317,7 @@ static void __init spectre_v2_select_mitigation(void)
* Retpoline means the kernel is safe because it has no indirect
* branches. But firmware isn't, so use IBRS to protect that.
*/
- if (boot_cpu_has(X86_FEATURE_IBRS)) {
+ if (mode != SPECTRE_V2_IBRS_ALL && boot_cpu_has(X86_FEATURE_IBRS)) {
setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
pr_info("Enabling Restricted Speculation for firmware calls\n");
}
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 91e3539..62ad7e3 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3419,13 +3419,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)

vmx->spec_ctrl = data;

- if (!data)
+ if (!data && !spectre_v2_ibrs_all())
break;

/*
* For non-nested:
* When it's written (to non-zero) for the first time, pass
- * it through.
+ * it through unless we have IBRS_ALL and it should just be
+ * set for ever.
*
* For nested:
* The handling of the MSR bitmap for L2 guests is done in
@@ -9441,7 +9442,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
* is no need to worry about the conditional branch over the wrmsr
* being speculatively taken.
*/
- if (vmx->spec_ctrl)
+ if (!spectre_v2_ibrs_all() && vmx->spec_ctrl)
wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);

vmx->__launched = vmx->loaded_vmcs->launched;
@@ -9563,11 +9564,12 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
);

/*
- * We do not use IBRS in the kernel. If this vCPU has used the
- * SPEC_CTRL MSR it may have left it on; save the value and
- * turn it off. This is much more efficient than blindly adding
- * it to the atomic save/restore list. Especially as the former
- * (Saving guest MSRs on vmexit) doesn't even exist in KVM.
+ * Without IBRS_ALL, we do not use IBRS in the kernel. If this
+ * vCPU has used the SPEC_CTRL MSR it may have left it on;
+ * save the value and turn it off. This is much more efficient
+ * than blindly adding it to the atomic save/restore list.
+ * Especially as the former (saving guest MSRs on vmexit)
+ * doesn't even exist in KVM.
*
* For non-nested case:
* If the L01 MSR bitmap does not intercept the MSR, then we need to
@@ -9576,12 +9578,17 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
* For nested case:
* If the L02 MSR bitmap does not intercept the MSR, then we need to
* save it.
+ *
+ * If IBRS_ALL is present then the whole thing is a no-op fiction
+ * for guests and every access is trapped, so do nothing.
*/
- if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
- rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+ if (!spectre_v2_ibrs_all()) {
+ if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
+ rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);

- if (vmx->spec_ctrl)
- wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+ if (vmx->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+ }

/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();
--
2.7.4