linux-kernel - [PATCH v2 7/7] x86/its: Allow stuffing in eIBRS+retpoline mode also

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Message-ID: <20250521-eibrs-fix-v2-7-70e2598e932c@linux.intel.com>
Date: Wed, 21 May 2025 19:45:55 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: x86@...nel.org
Cc: David Kaplan <david.kaplan@....com>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>
Subject: [PATCH v2 7/7] x86/its: Allow stuffing in eIBRS+retpoline mode also

After a recent restructuring of ITS mitigation, RSB stuffing can no
longer be enabled in eIBRS+Retpoline mode. Before ITS, retbleed
mitigation only allowed stuffing when eIBRS was not enabled. This was
perfectly fine since eIBRS mitigates retbleed.

However, RSB stuffing mitigation for ITS is still needed with eIBRS. The
restructuring solely relies on retbleed to deploy stuffing, and does not
allow it when eIBRS is enabled. This behavior is different from what was
before the restructuring. Fix it by allowing stuffing in eIBRS+retpoline
mode also.

Fixes: 8c57ca583ebf ("x86/bugs: Restructure ITS mitigation")
Closes: https://lore.kernel.org/lkml/20250519235101.2vm6sc5txyoykb2r@desk/
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 043da9eaf99c5e1f92ae6c56cb7f779bbceae0e0..ad04da8711b24f2d070e84275a77a4e561fe4d2a 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1130,7 +1130,8 @@ static inline bool cdt_possible(enum spectre_v2_mitigation mode)
 	    !IS_ENABLED(CONFIG_MITIGATION_RETPOLINE))
 		return false;
 
-	if (mode == SPECTRE_V2_RETPOLINE)
+	if (mode == SPECTRE_V2_RETPOLINE ||
+	    mode == SPECTRE_V2_EIBRS_RETPOLINE)
 		return true;
 
 	return false;
@@ -1280,7 +1281,7 @@ static void __init retbleed_update_mitigation(void)
 
 	if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF &&
 	    !cdt_possible(spectre_v2_enabled)) {
-		pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
+		pr_err("WARNING: retbleed=stuff depends on retpoline\n");
 		retbleed_mitigation = RETBLEED_MITIGATION_AUTO;
 	}
 
@@ -1457,6 +1458,7 @@ static void __init its_update_mitigation(void)
 		its_mitigation = ITS_MITIGATION_OFF;
 		break;
 	case SPECTRE_V2_RETPOLINE:
+	case SPECTRE_V2_EIBRS_RETPOLINE:
 		/* Retpoline+CDT mitigates ITS */
 		if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF)
 			its_mitigation = ITS_MITIGATION_RETPOLINE_STUFF;

-- 
2.34.1