x86/bugs: Fix spectre_v2 forcing

JIRA: https://issues.redhat.com/browse/RHEL-119227

commit 30ef245
Author: David Kaplan <david.kaplan@amd.com>
Date:   Mon, 15 Sep 2025 08:47:04 -0500

    x86/bugs: Fix spectre_v2 forcing

    There were two oddities with spectre_v2 command line options.

    First, any option other than 'off' or 'auto' would force spectre_v2
    mitigations even if the CPU (hypothetically) wasn't vulnerable to spectre_v2.
    That was inconsistent with all the other bugs where mitigations are ignored
    unless an explicit 'force' option is specified.

    Second, even though spectre_v2 mitigations would be enabled in these cases,
    the X86_BUG_SPECTRE_V2 bit wasn't set.  This is again inconsistent with the
    forcing behavior of other bugs and arguably incorrect as it doesn't make sense
    to enable a mitigation if the X86_BUG bit isn't set.

    Fix both issues by only forcing spectre_v2 mitigations when the
    'spectre_v2=on' option is specified (which was already called
    SPECTRE_V2_CMD_FORCE) and setting the relevant X86_BUG_* bits in that case.

    This also allows for simplifying bhi_update_mitigation() because
    spectre_v2_cmd will now always be SPECTRE_V2_CMD_NONE if the CPU is immune to
    spectre_v2.

    Signed-off-by: David Kaplan <david.kaplan@amd.com>
    Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
    Link: https://lore.kernel.org/20250915134706.3201818-1-david.kaplan@amd.com

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

arch/x86/kernel/cpu/bugs.c

@@ -2057,29 +2057,32 @@ static int __init spectre_v2_parse_cmdline(char *str)
 	if (nospectre_v2)
 		return 0;
 
-	if (!strcmp(str, "off"))
+	if (!strcmp(str, "off")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_NONE;
-	else if (!strcmp(str, "on"))
+	} else if (!strcmp(str, "on")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_FORCE;
-	else if (!strcmp(str, "retpoline"))
+		setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
+		setup_force_cpu_bug(X86_BUG_SPECTRE_V2_USER);
+	} else if (!strcmp(str, "retpoline")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_RETPOLINE;
-	else if (!strcmp(str, "retpoline,amd") ||
-		 !strcmp(str, "retpoline,lfence"))
+	} else if (!strcmp(str, "retpoline,amd") ||
+		 !strcmp(str, "retpoline,lfence")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_RETPOLINE_LFENCE;
-	else if (!strcmp(str, "retpoline,generic"))
+	} else if (!strcmp(str, "retpoline,generic")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_RETPOLINE_GENERIC;
-	else if (!strcmp(str, "eibrs"))
+	} else if (!strcmp(str, "eibrs")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_EIBRS;
-	else if (!strcmp(str, "eibrs,lfence"))
+	} else if (!strcmp(str, "eibrs,lfence")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_EIBRS_LFENCE;
-	else if (!strcmp(str, "eibrs,retpoline"))
+	} else if (!strcmp(str, "eibrs,retpoline")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_EIBRS_RETPOLINE;
-	else if (!strcmp(str, "auto"))
+	} else if (!strcmp(str, "auto")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_AUTO;
-	else if (!strcmp(str, "ibrs"))
+	} else if (!strcmp(str, "ibrs")) {
 		spectre_v2_cmd = SPECTRE_V2_CMD_IBRS;
-	else
+	} else {
 		pr_err("Ignoring unknown spectre_v2 option (%s).", str);
+	}
 
 	return 0;
 }
@@ -2232,10 +2235,6 @@ static void __init bhi_update_mitigation(void)
 {
 	if (spectre_v2_cmd == SPECTRE_V2_CMD_NONE)
 		bhi_mitigation = BHI_MITIGATION_OFF;
-
-	if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2) &&
-	     spectre_v2_cmd == SPECTRE_V2_CMD_AUTO)
-		bhi_mitigation = BHI_MITIGATION_OFF;
 }
 
 static void __init bhi_apply_mitigation(void)
@@ -2316,9 +2315,10 @@ static void __init spectre_v2_select_mitigation(void)
 		spectre_v2_cmd = SPECTRE_V2_CMD_AUTO;
 	}
 
-	if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2) &&
-	    (spectre_v2_cmd == SPECTRE_V2_CMD_NONE || spectre_v2_cmd == SPECTRE_V2_CMD_AUTO))
+	if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2)) {
+		spectre_v2_cmd = SPECTRE_V2_CMD_NONE;
 		return;
+	}
 
 	switch (spectre_v2_cmd) {
 	case SPECTRE_V2_CMD_NONE: