[PATCH] x86/bugs: Adapt SRSO mitigation to Zen6

[PATCH] x86/bugs: Adapt SRSO mitigation to Zen6

[Borislav Petkov]

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

Zen6 has BTB protection which isolates the different contexts
(user/kernel, guest/host) from one another. This makes the SafeRET
mitigation there unnecessary leaving the user/user and guest/guest
attack vectors open, whose protection is handled by the Spectre v2
mitigation setting to do IBPB on a context switch.

Detect that setting and report it with a new mitigation string.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/kernel/cpu/bugs.c         | 18 ++++++++++++++++--
 arch/x86/kernel/cpu/scattered.c    |  1 +
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 73d5c740202d..f838fc7af722 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -420,6 +420,7 @@
 #define X86_FEATURE_SUCCOR		(17*32+ 1) /* "succor" Uncorrectable error containment and recovery */
 #define X86_FEATURE_CPPC_PERF_PRIO	(17*32+ 2) /* CPPC Floor Perf support */
 #define X86_FEATURE_SMCA		(17*32+ 3) /* "smca" Scalable MCA */
+#define X86_FEATURE_BTB_CTX_ISOLATION	(17*32+ 4) /* AMD: Branch predictions contexts isolated */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX), word 18 */
 #define X86_FEATURE_AVX512_4VNNIW	(18*32+ 2) /* "avx512_4vnniw" AVX-512 Neural Network Instructions */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 56eac5611c31..1b2381da4d83 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1175,6 +1175,7 @@ enum srso_mitigation {
 	SRSO_MITIGATION_IBPB,
 	SRSO_MITIGATION_IBPB_ON_VMEXIT,
 	SRSO_MITIGATION_BP_SPEC_REDUCE,
+	SRSO_MITIGATION_USER_IBPB,
 };
 
 static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_AUTO;
@@ -2908,7 +2909,8 @@ static const char * const srso_strings[] = {
 	[SRSO_MITIGATION_SAFE_RET]		= "Mitigation: Safe RET",
 	[SRSO_MITIGATION_IBPB]			= "Mitigation: IBPB",
 	[SRSO_MITIGATION_IBPB_ON_VMEXIT]	= "Mitigation: IBPB on VMEXIT only",
-	[SRSO_MITIGATION_BP_SPEC_REDUCE]	= "Mitigation: Reduced Speculation"
+	[SRSO_MITIGATION_BP_SPEC_REDUCE]	= "Mitigation: Reduced Speculation",
+	[SRSO_MITIGATION_USER_IBPB]		= "Mitigation: IBPB on context switch",
 };
 
 static int __init srso_parse_cmdline(char *str)
@@ -2948,7 +2950,9 @@ static void __init srso_select_mitigation(void)
 		 * required.  Otherwise the 'microcode' mitigation is sufficient
 		 * to protect the user->user and guest->guest vectors.
 		 */
-		if (cpu_attack_vector_mitigated(CPU_MITIGATE_GUEST_HOST) ||
+		if ((cpu_attack_vector_mitigated(CPU_MITIGATE_GUEST_HOST) &&
+		    !boot_cpu_has(X86_FEATURE_BTB_CTX_ISOLATION))
+					||
 		    (cpu_attack_vector_mitigated(CPU_MITIGATE_USER_KERNEL) &&
 		     !boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))) {
 			srso_mitigation = SRSO_MITIGATION_SAFE_RET;
@@ -3024,6 +3028,16 @@ static void __init srso_update_mitigation(void)
 	    boot_cpu_has(X86_FEATURE_IBPB_BRTYPE))
 		srso_mitigation = SRSO_MITIGATION_IBPB;
 
+	/*
+	 * See if IBPB on context switch is the only thing needed to address
+	 * GUEST/GUEST and USER/USER vectors.
+	 */
+	if (srso_mitigation == SRSO_MITIGATION_MICROCODE  &&
+	    boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO) &&
+	    boot_cpu_has(X86_FEATURE_BTB_CTX_ISOLATION) &&
+	    spectre_v2_user_ibpb != SPECTRE_V2_USER_NONE)
+		srso_mitigation = SRSO_MITIGATION_USER_IBPB;
+
 	pr_info("%s\n", srso_strings[srso_mitigation]);
 }
 
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index 8665a6474806..41b4880be7ed 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -64,6 +64,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_AMD_WORKLOAD_CLASS,	CPUID_EAX, 22, 0x80000021, 0 },
 	{ X86_FEATURE_TSA_SQ_NO,		CPUID_ECX,  1, 0x80000021, 0 },
 	{ X86_FEATURE_TSA_L1_NO,		CPUID_ECX,  2, 0x80000021, 0 },
+	{ X86_FEATURE_BTB_CTX_ISOLATION,	CPUID_ECX,  8, 0x80000021, 0 },
 	{ X86_FEATURE_PERFMON_V2,		CPUID_EAX,  0, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_LBR_V2,		CPUID_EAX,  1, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_LBR_PMC_FREEZE,	CPUID_EAX,  2, 0x80000022, 0 },
-- 
2.53.0