[tip: x86/bugs] x86/bugs: Add SRSO_USER_KERNEL_NO support

[tip-bot2 for Borislav Petkov (AMD)]

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     877818802c3e970f67ccb53012facc78bef5f97a
Gitweb:        https://git.kernel.org/tip/877818802c3e970f67ccb53012facc78bef5f97a
Author:        Borislav Petkov (AMD) <bp@xxxxxxxxx>
AuthorDate:    Mon, 11 Nov 2024 17:22:08 +01:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Mon, 30 Dec 2024 17:48:33 +01:00

x86/bugs: Add SRSO_USER_KERNEL_NO support

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: Mitigation: IBPB on VMEXIT only

Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
Link: https://lore.kernel.org/r/20241202120416.6054-2-bp@xxxxxxxxxx
---
 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/kernel/cpu/bugs.c         | 4 ++++
 arch/x86/kernel/cpu/common.c       | 1 +
 3 files changed, 6 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 645aa36..0e2d817 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -465,6 +465,7 @@
 #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 47a01d4..5a505aa 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2615,6 +2615,9 @@ static void __init srso_select_mitigation(void)
 		break;
 
 	case SRSO_CMD_SAFE_RET:
+		if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
+			goto ibpb_on_vmexit;
+
 		if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
 			/*
 			 * Enable the return thunk for generated code
@@ -2658,6 +2661,7 @@ 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 3e90376..7e8d811 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),
 	{}
 };