Re: [PATCH] x86/bugs: KVM: Add support for SRSO_MSR_FIX

Sean Christopherson
2025-01-17T18:56:17Z

On Sat, Jan 11, 2025, Borislav Petkov wrote:
> Ok,
>
> here's a new version, I think I've addressed all outstanding review comments.
>
> Lemme know how we should proceed here, you take it or I? Judging by the
> diffstat probably I should and you ack it or so.

No preference, either way works for me.

> Also, lemme know if I should add your Co-developed-by for the user_return
> portion.

Heh, no preference again. In case you want to add Co-developed-by, here's my SoB:

Signed-off-by: Sean Christopherson <seanjc@google.com>

> In order to exploit vulnerability, an attacker needs to:
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 508c0dad116b..471447a31605 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -468,6 +468,7 @@
> #define X86_FEATURE_IBPB_BRTYPE (20*32+28) /* MSR_PRED_CMD[IBPB] flushes all branch type predictions */
> #define X86_FEATURE_SRSO_NO (20*32+29) /* CPU is not affected by SRSO */
> #define X86_FEATURE_SRSO_USER_KERNEL_NO (20*32+30) /* CPU is not affected by SRSO across user/kernel boundaries */
> +#define X86_FEATURE_SRSO_MSR_FIX (20*32+31) /* MSR BP_CFG[BpSpecReduce] can be used to mitigate SRSO for VMs */

Any objection to calling this X86_FEATURE_SRSO_BP_SPEC_REDUCE? More below.

> @@ -2540,12 +2541,19 @@ static const char * const srso_strings[] = {
> [SRSO_MITIGATION_MICROCODE] = "Vulnerable: Microcode, no safe RET",
> [SRSO_MITIGATION_SAFE_RET] = "Mitigation: Safe RET",
> [SRSO_MITIGATION_IBPB] = "Mitigation: IBPB",
> - [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only"
> + [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only",
> + [SRSO_MITIGATION_BP_SPEC_REDUCE] = "Mitigation: Reduced Speculation"
> };
>
> static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
> static enum srso_mitigation_cmd srso_cmd __ro_after_init = SRSO_CMD_SAFE_RET;
>
> +bool srso_spec_reduce_enabled(void)
> +{
> + return srso_mitigation == SRSO_MITIGATION_BP_SPEC_REDUCE;

At the risk of getting too clever, what if we use the X86_FEATURE to communicate
that KVM should toggle the magic MSR? That'd avoid the helper+export, and up to
this point "srso_mitigation" has been used only for documentation purposes.

The flag just needs to be cleared in two locations, which doesn't seem too awful.
Though if we go that route, SRSO_MSR_FIX is a pretty crappy name :-)

> diff --git a/arch/x86/lib/msr.c b/arch/x86/lib/msr.c
> index 4bf4fad5b148..5a18ecc04a6c 100644
> --- a/arch/x86/lib/msr.c
> +++ b/arch/x86/lib/msr.c
> @@ -103,6 +103,7 @@ int msr_set_bit(u32 msr, u8 bit)
> {
> return __flip_bit(msr, bit, true);
> }
> +EXPORT_SYMBOL_GPL(msr_set_bit);
>
> /**
> * msr_clear_bit - Clear @bit in a MSR @msr.
> @@ -118,6 +119,7 @@ int msr_clear_bit(u32 msr, u8 bit)
> {
> return __flip_bit(msr, bit, false);
> }
> +EXPORT_SYMBOL_GPL(msr_clear_bit);

These exports are no longer necessary.

Compile tested only...

---
Documentation/admin-guide/hw-vuln/srso.rst | 21 +++++++++++++++++++++
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/msr-index.h | 1 +
arch/x86/kernel/cpu/bugs.c | 15 ++++++++++++++-
arch/x86/kvm/svm/svm.c | 14 ++++++++++++++
5 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/hw-vuln/srso.rst b/Documentation/admin-guide/hw-vuln/srso.rst
index 2ad1c05b8c88..b856538083a2 100644
--- a/Documentation/admin-guide/hw-vuln/srso.rst
+++ b/Documentation/admin-guide/hw-vuln/srso.rst
@@ -104,6 +104,27 @@ The possible values in this file are:

(spec_rstack_overflow=ibpb-vmexit)

+ * 'Mitigation: Reduced Speculation':
+
+ This mitigation gets automatically enabled when the above one "IBPB on
+ VMEXIT" has been selected and the CPU supports the BpSpecReduce bit.
+
+ It gets automatically enabled on machines which have the
+ SRSO_USER_KERNEL_NO=1 CPUID bit. In that case, the code logic is to switch
+ to the above =ibpb-vmexit mitigation because the user/kernel boundary is
+ not affected anymore and thus "safe RET" is not needed.
+
+ After enabling the IBPB on VMEXIT mitigation option, the BpSpecReduce bit
+ is detected (functionality present on all such machines) and that
+ practically overrides IBPB on VMEXIT as it has a lot less performance
+ impact and takes care of the guest->host attack vector too.
+
+ Currently, the mitigation uses KVM's user_return approach
+ (kvm_set_user_return_msr()) to set the BpSpecReduce bit when a vCPU runs
+ a guest and reset it upon return to host userspace or when the KVM module
+ is unloaded. The intent being, the small perf impact of BpSpecReduce should
+ be incurred only when really necessary.
+

In order to exploit vulnerability, an attacker needs to:
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 0e2d81763615..00f2649c36ab 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -466,6 +466,7 @@
#define X86_FEATURE_IBPB_BRTYPE (20*32+28) /* MSR_PRED_CMD[IBPB] flushes all branch type predictions */
#define X86_FEATURE_SRSO_NO (20*32+29) /* CPU is not affected by SRSO */
#define X86_FEATURE_SRSO_USER_KERNEL_NO (20*32+30) /* CPU is not affected by SRSO across user/kernel boundaries */
+#define X86_FEATURE_SRSO_BP_SPEC_REDUCE (20*32+31) /* MSR BP_CFG[BpSpecReduce] can be used to mitigate SRSO for VMs */

/*
* Extended auxiliary flags: Linux defined - for features scattered in various
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 3ae84c3b8e6d..1372a569fb58 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -717,6 +717,7 @@

/* Zen4 */
#define MSR_ZEN4_BP_CFG 0xc001102e
+#define MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT 4
#define MSR_ZEN4_BP_CFG_SHARED_BTB_FIX_BIT 5

/* Fam 19h MSRs */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 8854d9bce2a5..971d3373eeaf 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2523,6 +2523,7 @@ enum srso_mitigation {
SRSO_MITIGATION_SAFE_RET,
SRSO_MITIGATION_IBPB,
SRSO_MITIGATION_IBPB_ON_VMEXIT,
+ SRSO_MITIGATION_BP_SPEC_REDUCE,
};

enum srso_mitigation_cmd {
@@ -2540,7 +2541,8 @@ static const char * const srso_strings[] = {
[SRSO_MITIGATION_MICROCODE] = "Vulnerable: Microcode, no safe RET",
[SRSO_MITIGATION_SAFE_RET] = "Mitigation: Safe RET",
[SRSO_MITIGATION_IBPB] = "Mitigation: IBPB",
- [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only"
+ [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only",
+ [SRSO_MITIGATION_BP_SPEC_REDUCE] = "Mitigation: Reduced Speculation"
};

static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
@@ -2577,6 +2579,8 @@ static void __init srso_select_mitigation(void)
if (!boot_cpu_has_bug(X86_BUG_SRSO) ||
cpu_mitigations_off() ||
srso_cmd == SRSO_CMD_OFF) {
+ setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);
+
if (boot_cpu_has(X86_FEATURE_SBPB))
x86_pred_cmd = PRED_CMD_SBPB;
return;
@@ -2665,6 +2669,12 @@ static void __init srso_select_mitigation(void)

ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
+ if (boot_cpu_has(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
+ pr_notice("Reducing speculation to address VM/HV SRSO attack vector.\n");
+ srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE;
+ break;
+ }
+
if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
@@ -2686,6 +2696,9 @@ static void __init srso_select_mitigation(void)
}

out:
+ if (srso_mitigation != SRSO_MITIGATION_BP_SPEC_REDUCE)
+ setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);
+
pr_info("%s\n", srso_strings[srso_mitigation]);
}

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 7640a84e554a..6ea3632af580 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -257,6 +257,7 @@ DEFINE_PER_CPU(struct svm_cpu_data, svm_data);
* defer the restoration of TSC_AUX until the CPU returns to userspace.
*/
static int tsc_aux_uret_slot __read_mostly = -1;
+static int zen4_bp_cfg_uret_slot __ro_after_init = -1;

static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};

@@ -1540,6 +1541,11 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
(!boot_cpu_has(X86_FEATURE_V_TSC_AUX) || !sev_es_guest(vcpu->kvm)))
kvm_set_user_return_msr(tsc_aux_uret_slot, svm->tsc_aux, -1ull);

+ if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
+ kvm_set_user_return_msr(zen4_bp_cfg_uret_slot,
+ BIT_ULL(MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT),
+ BIT_ULL(MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT));
+
svm->guest_state_loaded = true;
}

@@ -5306,6 +5312,14 @@ static __init int svm_hardware_setup(void)

tsc_aux_uret_slot = kvm_add_user_return_msr(MSR_TSC_AUX);

+ if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
+ zen4_bp_cfg_uret_slot = kvm_add_user_return_msr(MSR_ZEN4_BP_CFG);
+ if (WARN_ON_ONCE(zen4_bp_cfg_uret_slot < 0)) {
+ r = -EIO;
+ goto err;
+ }
+ }
+
if (boot_cpu_has(X86_FEATURE_AUTOIBRS))
kvm_enable_efer_bits(EFER_AUTOIBRS);

base-commit: 9bec4a1f4ea92b99f96894e0c51c192b5345aa91
--