[PATCH v2 1/4] x86/bugs: Add SRSO_USER_KERNEL_NO support

Borislav Petkov
2024-12-02T12:04:23Z

From: "Borislav Petkov (AMD)" <bp@alien8.de>

If the machine has:

CPUID Fn8000_0021_EAX[30] (SRSO_USER_KERNEL_NO) -- If this bit is 1,
it indicates the CPU is not subject to the SRSO vulnerability across
user/kernel boundaries.

have it fall back to IBPB on VMEXIT only, in the case it is going to run
VMs:

Speculative Return Stack Overflow: CPU user/kernel transitions protected, falling back to IBPB-on-VMEXIT
Speculative Return Stack Overflow: Mitigation: IBPB on VMEXIT only

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/bugs.c | 6 ++++++
arch/x86/kernel/cpu/common.c | 1 +
3 files changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 17b6590748c0..2787227a8b42 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -464,6 +464,7 @@
#define X86_FEATURE_SBPB (20*32+27) /* Selective Branch Prediction Barrier */
#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 */

/*
* Extended auxiliary flags: Linux defined - for features scattered in various
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47a01d4028f6..8854d9bce2a5 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2615,6 +2615,11 @@ static void __init srso_select_mitigation(void)
break;

case SRSO_CMD_SAFE_RET:
+ if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO)) {
+ pr_notice("CPU user/kernel transitions protected, falling back to IBPB-on-VMEXIT\n");
+ goto ibpb_on_vmexit;
+ }
+
if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
/*
* Enable the return thunk for generated code
@@ -2658,6 +2663,7 @@ static void __init srso_select_mitigation(void)
}
break;

+ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a5c28975c608..954f9c727f11 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1270,6 +1270,7 @@ static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
VULNBL_AMD(0x17, RETBLEED | SMT_RSB | SRSO),
VULNBL_HYGON(0x18, RETBLEED | SMT_RSB | SRSO),
VULNBL_AMD(0x19, SRSO),
+ VULNBL_AMD(0x1a, SRSO),
{}
};

--
2.43.0