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

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

[Borislav Petkov]

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

RE: [PATCH] x86/bugs: Fix SRSO reporting on Zen1/2 with SMT disabled

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Borislav Petkov <bp@kernel.org>
> Sent: Tuesday, May 13, 2025 6:04 AM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: X86 ML <x86@kernel.org>; LKML <linux-kernel@vger.kernel.org>; Borislav
> Petkov (AMD) <bp@alien8.de>
> Subject: [PATCH] x86/bugs: Fix SRSO reporting on Zen1/2 with SMT disabled
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> 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

LGTM.

Reviewed-by: David Kaplan <david.kaplan@amd.com>

--David Kaplan

[tip: x86/core] x86/bugs: Fix SRSO reporting on Zen1/2 with SMT disabled

[tip-bot2 for Borislav Petkov (AMD)]

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]);
 }