linux-kernel - [PATCH 16/29] x86/bugs: Disable Retpoline when IBT

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Date:   Fri, 18 Feb 2022 17:49:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com
Subject: [PATCH 16/29] x86/bugs: Disable Retpoline when IBT

Retpoline and IBT are mutually exclusive. IBT relies on indirect
branches (JMP/CALL *%reg) while retpoline avoids them by design.

Demote to LFENCE on IBT enabled hardware.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/cpu/bugs.c |   25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -937,6 +937,11 @@ static void __init spectre_v2_select_mit
 	    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
 	retpoline_amd:
 		if (!boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
+			if (IS_ENABLED(CONFIG_X86_IBT) &&
+			    boot_cpu_has(X86_FEATURE_IBT)) {
+				pr_err("Spectre mitigation: LFENCE not serializing, generic retpoline not available due to IBT, switching to none\n");
+				return;
+			}
 			pr_err("Spectre mitigation: LFENCE not serializing, switching to generic retpoline\n");
 			goto retpoline_generic;
 		}
@@ -945,6 +950,26 @@ static void __init spectre_v2_select_mit
 		setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
 	} else {
 	retpoline_generic:
+		/*
+		 *  Full retpoline is incompatible with IBT, demote to LFENCE.
+		 */
+		if (IS_ENABLED(CONFIG_X86_IBT) &&
+		    boot_cpu_has(X86_FEATURE_IBT)) {
+			switch (cmd) {
+			case SPECTRE_V2_CMD_FORCE:
+			case SPECTRE_V2_CMD_AUTO:
+			case SPECTRE_V2_CMD_RETPOLINE:
+				/* silent for auto select */
+				break;
+
+			default:
+				/* warn when 'demoting' an explicit selection */
+				pr_warn("Spectre mitigation: Switching to LFENCE due to IBT\n");
+				break;
+			}
+
+			goto retpoline_amd;
+		}
 		mode = SPECTRE_V2_RETPOLINE_GENERIC;
 		setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
 	}