linux-kernel - [PATCH v2] x86: Clear feature bits disabled at compile-time

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Message-ID: <20250723092250.3411923-1-maciej.wieczor-retman@intel.com>
Date: Wed, 23 Jul 2025 11:22:49 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Kirill A. Shutemov" <kas@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Xin Li <xin3.li@...el.com>,
	Sai Praneeth <sai.praneeth.prakhya@...el.com>,
	Jethro Beekman <jethro@...tanix.com>,
	Jarkko Sakkinen <jarkko@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Kees Cook <kees@...nel.org>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc: maciej.wieczor-retman@...el.com,
	stable@...r.kernel.org,
	Borislav Petkov <bp@...e.de>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] x86: Clear feature bits disabled at compile-time

If some config options are disabled during compile time, they still are
enumerated in macros that use the x86_capability bitmask - cpu_has() or
this_cpu_has().

The features are also visible in /proc/cpuinfo even though they are not
enabled - which is contrary to what the documentation states about the
file. Examples of such feature flags are lam, fred, sgx, ibrs_enhanced,
split_lock_detect, user_shstk, avx_vnni and enqcmd.

Add a DISABLED_MASK() macro that returns 32 bit chunks of the disabled
feature bits bitmask.

Initialize the cpu_caps_cleared and cpu_caps_set arrays with the
contents of the disabled and required bitmasks respectively. Then let
apply_forced_caps() clear/set these feature bits in the x86_capability.

Fixes: 6449dcb0cac7 ("x86: CPUID and CR3/CR4 flags for Linear Address Masking")
Fixes: 51c158f7aacc ("x86/cpufeatures: Add the CPU feature bit for FRED")
Fixes: 706d51681d63 ("x86/speculation: Support Enhanced IBRS on future CPUs")
Fixes: e7b6385b01d8 ("x86/cpufeatures: Add Intel SGX hardware bits")
Fixes: 6650cdd9a8cc ("x86/split_lock: Enable split lock detection by kernel")
Fixes: 701fb66d576e ("x86/cpufeatures: Add CPU feature flags for shadow stacks")
Fixes: ff4f82816dff ("x86/cpufeatures: Enumerate ENQCMD and ENQCMDS instructions")
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Cc: <stable@...r.kernel.org>
---
Changelog v2:
- Redo the patch to utilize a more generic solution, not just fix the
  LAM and FRED feature bits.
- Note more feature flags that shouldn't be present.
- Add fixes and cc tags.

 arch/x86/kernel/cpu/common.c       | 12 ++++++++++++
 arch/x86/tools/cpufeaturemasks.awk |  8 ++++++++
 2 files changed, 20 insertions(+)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 77afca95cced..ba8b5fba8552 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1709,6 +1709,16 @@ static void __init cpu_parse_early_param(void)
 	}
 }
 
+static __init void init_cpu_cap(struct cpuinfo_x86 *c)
+{
+	int i;
+
+	for (i = 0; i < NCAPINTS; i++) {
+		cpu_caps_set[i] = REQUIRED_MASK(i);
+		cpu_caps_cleared[i] = DISABLED_MASK(i);
+	}
+}
+
 /*
  * Do minimum CPU detection early.
  * Fields really needed: vendor, cpuid_level, family, model, mask,
@@ -1782,6 +1792,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 	if (!pgtable_l5_enabled())
 		setup_clear_cpu_cap(X86_FEATURE_LA57);
 
+	init_cpu_cap(c);
+
 	detect_nopl();
 }
 
diff --git a/arch/x86/tools/cpufeaturemasks.awk b/arch/x86/tools/cpufeaturemasks.awk
index 173d5bf2d999..2e2412f7681f 100755
--- a/arch/x86/tools/cpufeaturemasks.awk
+++ b/arch/x86/tools/cpufeaturemasks.awk
@@ -82,6 +82,14 @@ END {
 		}
 		printf " 0\t\\\n";
 		printf "\t) & (1U << ((x) & 31)))\n\n";
+
+		printf "\n#define %s_MASK(x)\t\t\t\t\\\n", s;
+		printf "\t((\t\t\t\t";
+		for (i = 0; i < ncapints; i++) {
+			if (masks[i])
+				printf "\t\t\\\n\t\t(x) == %2d ? %s_MASK%d :", i, s, i;
+		}
+		printf " 0))\t\\\n\n";
 	}
 
 	printf "#endif /* _ASM_X86_CPUFEATUREMASKS_H */\n";
-- 
2.49.0