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

1f4bb068b498 ("x86/bugs: Restructure SRSO mitigation") does this:

  if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
          setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
          srso_mitigation = SRSO_MITIGATION_NONE;
          return;
  }

and, in particular, sets srso_mitigation to NONE. This leads to
reporting

  Speculative Return Stack Overflow: Vulnerable

on Zen2 machines.

There's a far bigger confusion with what SRSO_NO means and how it is
used in the code but this will be a matter of future fixes and
restructuring to how the SRSO mitigation gets determined.

Fix the reporting issue for now.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: David Kaplan <david.kaplan@amd.com>
---
 arch/x86/kernel/cpu/bugs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47c74c4ae206..dd8b50b4ceaa 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2942,7 +2942,9 @@ static void __init srso_update_mitigation(void)
 	    boot_cpu_has(X86_FEATURE_IBPB_BRTYPE))
 		srso_mitigation = SRSO_MITIGATION_IBPB;
 
-	if (boot_cpu_has_bug(X86_BUG_SRSO) && !cpu_mitigations_off())
+	if (boot_cpu_has_bug(X86_BUG_SRSO) &&
+	    !cpu_mitigations_off() &&
+	    !boot_cpu_has(X86_FEATURE_SRSO_NO))
 		pr_info("%s\n", srso_strings[srso_mitigation]);
 }
 
-- 
2.43.0

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

Commit-ID:     891d3b8be32a69ae94d32e3f1e1ee01359020d49
Gitweb:        https://git.kernel.org/tip/891d3b8be32a69ae94d32e3f1e1ee01359020d49
Author:        Borislav Petkov (AMD) <bp@alien8.de>
AuthorDate:    Tue, 13 May 2025 13:04:05 +02:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Tue, 13 May 2025 22:31:08 +02:00

x86/bugs: Fix SRSO reporting on Zen1/2 with SMT disabled

1f4bb068b498 ("x86/bugs: Restructure SRSO mitigation") does this:

  if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
          setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
          srso_mitigation = SRSO_MITIGATION_NONE;
          return;
  }

and, in particular, sets srso_mitigation to NONE. This leads to
reporting

  Speculative Return Stack Overflow: Vulnerable

on Zen2 machines.

There's a far bigger confusion with what SRSO_NO means and how it is
used in the code but this will be a matter of future fixes and
restructuring to how the SRSO mitigation gets determined.

Fix the reporting issue for now.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: David Kaplan <david.kaplan@amd.com>
Link: https://lore.kernel.org/20250513110405.15872-1-bp@kernel.org
---
 arch/x86/kernel/cpu/bugs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47c74c4..dd8b50b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2942,7 +2942,9 @@ static void __init srso_update_mitigation(void)
 	    boot_cpu_has(X86_FEATURE_IBPB_BRTYPE))
 		srso_mitigation = SRSO_MITIGATION_IBPB;
 
-	if (boot_cpu_has_bug(X86_BUG_SRSO) && !cpu_mitigations_off())
+	if (boot_cpu_has_bug(X86_BUG_SRSO) &&
+	    !cpu_mitigations_off() &&
+	    !boot_cpu_has(X86_FEATURE_SRSO_NO))
 		pr_info("%s\n", srso_strings[srso_mitigation]);
 }