linux-kernel - [PATCH RFC v1 09/11] x86/fpu/apx: Disallow conflicting MPX presence

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Message-ID: <20250227184502.10288-10-chang.seok.bae@intel.com>
Date: Thu, 27 Feb 2025 10:44:54 -0800
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	chang.seok.bae@...el.com
Subject: [PATCH RFC v1 09/11] x86/fpu/apx: Disallow conflicting MPX presence

APX is introduced as xstate component 19, following AMX. However, its
offset in the non-compacted format overlaps with the area previously
occupied by the now-removed MPX. As they cannot coexist, MPX support was
removed from the kernel.

Despite this, the kernel must ensure the CPU never expose both features
at the same time. If so, it indicates unreliable hardware. In such cases,
XSAVE should be disabled entirely as a precautionary measure.

Add a sanity check to detect this condition and disable XSAVE if an
invalid hardware configuration is identified.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
---
 arch/x86/kernel/fpu/xstate.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 6a6f0e78e2c3..0f731e11c414 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -802,6 +802,17 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 		goto out_disable;
 	}
 
+	if (fpu_kernel_cfg.max_features & XFEATURE_MASK_APX &&
+	    fpu_kernel_cfg.max_features & XFEATURE_MASK_BNDREGS) {
+		/*
+		 * This is a problematic CPU configuration where two
+		 * conflicting state components are both enumerated.
+		 */
+		pr_err("x86/fpu: both APX and MPX present in the CPU's xstate features: 0x%llx.\n",
+		       fpu_kernel_cfg.max_features);
+		goto out_disable;
+	}
+
 	fpu_kernel_cfg.independent_features = fpu_kernel_cfg.max_features &
 					      XFEATURE_MASK_INDEPENDENT;
 
-- 
2.45.2