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Message-ID: <20250827153358.GJaK8lZm3cggYDbw2C@fat_crate.local>
Date: Wed, 27 Aug 2025 17:33:58 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Kaplan, David" <David.Kaplan@....com>
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"x86@...nel.org" <x86@...nel.org>,
"H . Peter Anvin" <hpa@...or.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] x86/bugs: Add attack vector controls for SSB
On Wed, Aug 27, 2025 at 02:25:26PM +0000, Kaplan, David wrote:
> Ok. Then I would go with the suggestion in my reply...move the
> should_mitigate_vuln() logic into the SPEC_STORE_BYPASS_CMD_AUTO branch of
> the switch. I think that should work as expected.
Makes sense...
> Rest of the patch was fine I think.
Here it is - I *think* it looks good now but doublecheck me again pls.
Thx.
---
From: David Kaplan <david.kaplan@....com>
Date: Tue, 19 Aug 2025 14:21:59 -0500
Subject: [PATCH] x86/bugs: Add attack vector controls for SSB
Attack vector controls for SSB were missed in the initial attack vector series.
The default mitigation for SSB requires user-space opt-in so it is only
relevant for user->user attacks. Add an AUTO mitigation for SSB and use this
attack vector control to select the SSB mitigation.
Fixes: 2d31d2874663 ("x86/bugs: Define attack vectors relevant for each bug")
Signed-off-by: David Kaplan <david.kaplan@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/20250819192200.2003074-5-david.kaplan@amd.com
---
.../admin-guide/hw-vuln/attack_vector_controls.rst | 5 +----
arch/x86/include/asm/nospec-branch.h | 1 +
arch/x86/kernel/cpu/bugs.c | 13 ++++++++++++-
3 files changed, 14 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/hw-vuln/attack_vector_controls.rst b/Documentation/admin-guide/hw-vuln/attack_vector_controls.rst
index 6dd0800146f6..5964901d66e3 100644
--- a/Documentation/admin-guide/hw-vuln/attack_vector_controls.rst
+++ b/Documentation/admin-guide/hw-vuln/attack_vector_controls.rst
@@ -215,7 +215,7 @@ Spectre_v2 X X
Spectre_v2_user X X * (Note 1)
SRBDS X X X X
SRSO X X X X
-SSB (Note 4)
+SSB X
TAA X X X X * (Note 2)
TSA X X X X
=============== ============== ============ ============= ============== ============ ========
@@ -229,9 +229,6 @@ Notes:
3 -- Disables SMT if cross-thread mitigations are fully enabled, the CPU is
vulnerable, and STIBP is not supported
- 4 -- Speculative store bypass is always enabled by default (no kernel
- mitigation applied) unless overridden with spec_store_bypass_disable option
-
When an attack-vector is disabled, all mitigations for the vulnerabilities
listed in the above table are disabled, unless mitigation is required for a
different enabled attack-vector or a mitigation is explicitly selected via a
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 10f261678749..e263c126723a 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -514,6 +514,7 @@ enum spectre_v2_user_mitigation {
/* The Speculative Store Bypass disable variants */
enum ssb_mitigation {
SPEC_STORE_BYPASS_NONE,
+ SPEC_STORE_BYPASS_AUTO,
SPEC_STORE_BYPASS_DISABLE,
SPEC_STORE_BYPASS_PRCTL,
SPEC_STORE_BYPASS_SECCOMP,
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 49ef1b832c1a..7b4b43aabd18 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -416,6 +416,10 @@ static bool __init should_mitigate_vuln(unsigned int bug)
cpu_attack_vector_mitigated(CPU_MITIGATE_USER_USER) ||
cpu_attack_vector_mitigated(CPU_MITIGATE_GUEST_GUEST) ||
(smt_mitigations != SMT_MITIGATIONS_OFF);
+
+ case X86_BUG_SPEC_STORE_BYPASS:
+ return cpu_attack_vector_mitigated(CPU_MITIGATE_USER_USER);
+
default:
WARN(1, "Unknown bug %x\n", bug);
return false;
@@ -2619,7 +2623,8 @@ void cpu_bugs_smt_update(void)
#undef pr_fmt
#define pr_fmt(fmt) "Speculative Store Bypass: " fmt
-static enum ssb_mitigation ssb_mode __ro_after_init = SPEC_STORE_BYPASS_NONE;
+static enum ssb_mitigation ssb_mode __ro_after_init =
+ IS_ENABLED(CONFIG_MITIGATION_SSB) ? SPEC_STORE_BYPASS_AUTO : SPEC_STORE_BYPASS_NONE;
/* The kernel command line selection */
enum ssb_mitigation_cmd {
@@ -2710,6 +2715,11 @@ static void __init ssb_select_mitigation(void)
ssb_mode = SPEC_STORE_BYPASS_DISABLE;
break;
case SPEC_STORE_BYPASS_CMD_AUTO:
+ if (should_mitigate_vuln(X86_BUG_SPEC_STORE_BYPASS))
+ ssb_mode = SPEC_STORE_BYPASS_PRCTL;
+ else
+ ssb_mode = SPEC_STORE_BYPASS_NONE;
+ break;
case SPEC_STORE_BYPASS_CMD_PRCTL:
ssb_mode = SPEC_STORE_BYPASS_PRCTL;
break;
@@ -2935,6 +2945,7 @@ static int ssb_prctl_get(struct task_struct *task)
return PR_SPEC_DISABLE;
case SPEC_STORE_BYPASS_SECCOMP:
case SPEC_STORE_BYPASS_PRCTL:
+ case SPEC_STORE_BYPASS_AUTO:
if (task_spec_ssb_force_disable(task))
return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
if (task_spec_ssb_noexec(task))
--
2.51.0
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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