[PATCH 6.12 087/105] x86/bugs: Add SRSO_USER_KERNEL_NO support

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit 877818802c3e970f67ccb53012facc78bef5f97a upstream.

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@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20241202120416.6054-2-bp@kernel.org
[ Harshit: Conflicts resolved as this commit: 7c62c442b6eb ("x86/vmscape:
  Enumerate VMSCAPE bug") has been applied already to 6.12.y ]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/include/asm/cpufeatures.h |    1 +
 arch/x86/kernel/cpu/bugs.c         |    4 ++++
 2 files changed, 5 insertions(+)

--- 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
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2810,6 +2810,9 @@ static void __init srso_select_mitigatio
 		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
@@ -2861,6 +2864,7 @@ static void __init srso_select_mitigatio
 		}
 		break;
 
+ibpb_on_vmexit:
 	case SRSO_CMD_IBPB_ON_VMEXIT:
 		if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
 			if (has_microcode) {