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.  Check with attack vector controls when
the command is auto - i.e., no explicit user selection has been done.

Fixes: 2d31d28 ("x86/bugs: Define attack vectors relevant for each bug")
Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250819192200.2003074-5-david.kaplan@amd.com

Author: dkaplan2

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

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;
@@ -2710,6 +2714,11 @@ 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;