linux-kernel - [PATCH v3 4/6] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

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Message-ID: <a305206cd08cde28c46a1ef19f5668a3fff9b013.1743617897.git.jpoimboe@kernel.org>
Date: Wed,  2 Apr 2025 11:19:21 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	amit@...nel.org,
	kvm@...r.kernel.org,
	amit.shah@....com,
	thomas.lendacky@....com,
	bp@...en8.de,
	tglx@...utronix.de,
	peterz@...radead.org,
	pawan.kumar.gupta@...ux.intel.com,
	corbet@....net,
	mingo@...hat.com,
	dave.hansen@...ux.intel.com,
	hpa@...or.com,
	seanjc@...gle.com,
	pbonzini@...hat.com,
	daniel.sneddon@...ux.intel.com,
	kai.huang@...el.com,
	sandipan.das@....com,
	boris.ostrovsky@...cle.com,
	Babu.Moger@....com,
	david.kaplan@....com,
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	andrew.cooper3@...rix.com
Subject: [PATCH v3 4/6] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

eIBRS protects against guest->host RSB underflow/poisoning attacks.
Adding retpoline to the mix doesn't change that.  Retpoline has a
balanced CALL/RET anyway.

So the current full RSB filling on VMEXIT with eIBRS+retpoline is
overkill.  Disable it or do the VMEXIT_LITE mitigation if needed.

Suggested-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Reviewed-by: Amit Shah <amit.shah@....com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9f9637cff7a3..354411fd4800 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1617,20 +1617,20 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	case SPECTRE_V2_NONE:
 		return;
 
-	case SPECTRE_V2_EIBRS_LFENCE:
 	case SPECTRE_V2_EIBRS:
+	case SPECTRE_V2_EIBRS_LFENCE:
+	case SPECTRE_V2_EIBRS_RETPOLINE:
 		if (boot_cpu_has_bug(X86_BUG_EIBRS_PBRSB)) {
-			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 			pr_info("Spectre v2 / PBRSB-eIBRS: Retire a single CALL on VMEXIT\n");
+			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 		}
 		return;
 
-	case SPECTRE_V2_EIBRS_RETPOLINE:
 	case SPECTRE_V2_RETPOLINE:
 	case SPECTRE_V2_LFENCE:
 	case SPECTRE_V2_IBRS:
-		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
+		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		return;
 	}
 
-- 
2.48.1