linux-kernel - [PATCH 1/2] x86: Use CPUID 0x8000001f to confirm SME availability

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Date:   Thu, 10 Feb 2022 23:36:51 -0600
From:   Mario Limonciello <mario.limonciello@....com>
To:     Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
CC:     Kees Cook <keescook@...omium.org>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        <hughsient@...il.com>,
        Martin Fernandez <martin.fernandez@...ypsium.com>,
        <linux-kernel@...r.kernel.org>,
        "Mario Limonciello" <mario.limonciello@....com>
Subject: [PATCH 1/2] x86: Use CPUID 0x8000001f to confirm SME availability

An upcoming change will disable the X86 SME feature flag when the
kernel hasn't activated SME.  Avoid relying upon that when determining
whether to call `native_wbinvd` by directly calling the CPUID that
indicates it.

Suggested-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Mario Limonciello <mario.limonciello@....com>
---
 arch/x86/kernel/process.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 81d8ef036637..e131d71b3cae 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -765,8 +765,11 @@ void stop_this_cpu(void *dummy)
 	 * without the encryption bit, they don't race each other when flushed
 	 * and potentially end up with the wrong entry being committed to
 	 * memory.
+	 *
+	 * Test the CPUID bit directly because the machine might've cleared
+	 * X86_FEATURE_SME due to cmdline options.
 	 */
-	if (boot_cpu_has(X86_FEATURE_SME))
+	if (cpuid_eax(0x8000001f) & BIT(0))
 		native_wbinvd();
 	for (;;) {
 		/*
-- 
2.34.1