linux-kernel - [PATCH v2 11/14] x86: LAM initialization

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Message-ID: <e1a92159d657d1f389a8ee799a33ee2173098bb6.1739866028.git.maciej.wieczor-retman@intel.com>
Date: Tue, 18 Feb 2025 09:15:27 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: [PATCH v2 11/14] x86: LAM initialization

To make use of KASAN's tag based mode on x86 Linear Address Masking
(LAM) needs to be enabled. To do that the 28th bit in CR4 needs to be
set.

Set the bit in early memory initialization.

When launching secondary CPUs the LAM bit gets lost. To avoid this it
needs to get added in a mask in head_64.S. The bit mask permits some
bits of CR4 to pass from the primary CPU to the secondary CPUs without
being cleared.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
---
 arch/x86/kernel/head_64.S | 3 +++
 arch/x86/mm/init.c        | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 31345e0ba006..87158729f138 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -206,6 +206,9 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
 	 *  there will be no global TLB entries after the execution."
 	 */
 	movl	$(X86_CR4_PAE | X86_CR4_LA57), %edx
+#ifdef CONFIG_ADDRESS_MASKING
+	orl	$X86_CR4_LAM_SUP, %edx
+#endif
 #ifdef CONFIG_X86_MCE
 	/*
 	 * Preserve CR4.MCE if the kernel will enable #MC support.
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 62aa4d66a032..5499ba683b53 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -761,6 +761,9 @@ void __init init_mem_mapping(void)
 	probe_page_size_mask();
 	setup_pcid();
 
+	if (boot_cpu_has(X86_FEATURE_LAM) && IS_ENABLED(CONFIG_KASAN_SW_TAGS))
+		cr4_set_bits_and_update_boot(X86_CR4_LAM_SUP);
+
 #ifdef CONFIG_X86_64
 	end = max_pfn << PAGE_SHIFT;
 #else
-- 
2.47.1