linux-kernel - [tip: x86/bugs] x86/bugs: Allow ITS stuffing in eIBRS+retpoline mode also

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Message-ID: <175075954520.406.3211204813089826157.tip-bot2@tip-bot2>
Date: Tue, 24 Jun 2025 10:05:45 -0000
From: "tip-bot2 for Pawan Gupta" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: x86/bugs] x86/bugs: Allow ITS stuffing in eIBRS+retpoline mode also

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     e88b1627b86eb756406ca0738c3db86351a58e4f
Gitweb:        https://git.kernel.org/tip/e88b1627b86eb756406ca0738c3db86351a58e4f
Author:        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
AuthorDate:    Wed, 11 Jun 2025 10:30:33 -07:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Mon, 23 Jun 2025 12:29:49 +02:00

x86/bugs: Allow ITS stuffing in eIBRS+retpoline mode also

After a recent restructuring of the 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>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/20250611-eibrs-fix-v4-7-5ff86cac6c61@linux.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 31f3db0..bdef2c9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1126,7 +1126,8 @@ 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;
@@ -1281,7 +1282,7 @@ 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_NONE;
 	}
 
@@ -1454,6 +1455,7 @@ 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;