[RFC PATCH v2 5/6] x86/boot: Drop the early variant of pgtable_l5_enabled()

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Now that cpu_feature_enabled(X86_FEATURE_5LEVEL_PAGING) is guaranteed to
produce the correct value even during early boot, there is no longer a
need for an early variant and so it can be dropped.

For the decompressor, fall back to testing the CR4.LA57 control register
bit directly.

Note that this removes the need to disable KASAN temporarily while
applying alternatives, given that any constant or VA space dimension
derived from pgtable_l5_enabled() will now always produce a consistent
value.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/compressed/misc.h         |  7 ++++---
 arch/x86/boot/startup/sme.c             |  9 ---------
 arch/x86/include/asm/pgtable_64_types.h | 14 ++------------
 arch/x86/kernel/alternative.c           | 12 ------------
 arch/x86/kernel/cpu/common.c            |  2 --
 arch/x86/kernel/head64.c                |  3 ---
 arch/x86/mm/kasan_init_64.c             |  3 ---
 7 files changed, 6 insertions(+), 44 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index db1048621ea2..72b830b8a69c 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -16,9 +16,6 @@
 
 #define __NO_FORTIFY
 
-/* cpu_feature_enabled() cannot be used this early */
-#define USE_EARLY_PGTABLE_L5
-
 /*
  * Boot stub deals with identity mappings, physical and virtual addresses are
  * the same, so override these defines.
@@ -28,6 +25,10 @@
 #define __pa(x)  ((unsigned long)(x))
 #define __va(x)  ((void *)((unsigned long)(x)))
 
+#ifdef CONFIG_X86_5LEVEL
+#define pgtable_l5_enabled() (native_read_cr4() & X86_CR4_LA57)
+#endif
+
 #include <linux/linkage.h>
 #include <linux/screen_info.h>
 #include <linux/elf.h>
diff --git a/arch/x86/boot/startup/sme.c b/arch/x86/boot/startup/sme.c
index 753cd2094080..c791f6b8a92f 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -25,15 +25,6 @@
 #undef CONFIG_PARAVIRT_XXL
 #undef CONFIG_PARAVIRT_SPINLOCKS
 
-/*
- * This code runs before CPU feature bits are set. By default, the
- * pgtable_l5_enabled() function uses bit X86_FEATURE_LA57 to determine if
- * 5-level paging is active, so that won't work here. USE_EARLY_PGTABLE_L5
- * is provided to handle this situation and, instead, use a variable that
- * has been set by the early boot code.
- */
-#define USE_EARLY_PGTABLE_L5
-
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/mem_encrypt.h>
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 88dc719b7d37..83cd6c4b9a3f 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -24,19 +24,9 @@ typedef struct { pmdval_t pmd; } pmd_t;
 extern unsigned int __pgtable_l5_enabled;
 
 #ifdef CONFIG_X86_5LEVEL
-#ifdef USE_EARLY_PGTABLE_L5
-/*
- * cpu_feature_enabled() is not available in early boot code.
- * Use variable instead.
- */
-static inline bool pgtable_l5_enabled(void)
-{
-	return __pgtable_l5_enabled;
-}
-#else
+#ifndef pgtable_l5_enabled
 #define pgtable_l5_enabled() cpu_feature_enabled(X86_FEATURE_5LEVEL_PAGING)
-#endif /* USE_EARLY_PGTABLE_L5 */
-
+#endif
 #else
 #define pgtable_l5_enabled() 0
 #endif /* CONFIG_X86_5LEVEL */
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index bf82c6f7d690..f4a8b81aac43 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -456,16 +456,6 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 
 	DPRINTK(ALT, "alt table %px, -> %px", start, end);
 
-	/*
-	 * In the case CONFIG_X86_5LEVEL=y, KASAN_SHADOW_START is defined using
-	 * cpu_feature_enabled(X86_FEATURE_LA57) and is therefore patched here.
-	 * During the process, KASAN becomes confused seeing partial LA57
-	 * conversion and triggers a false-positive out-of-bound report.
-	 *
-	 * Disable KASAN until the patching is complete.
-	 */
-	kasan_disable_current();
-
 	/*
 	 * The scan order should be from start to end. A later scanned
 	 * alternative code can overwrite previously scanned alternative code.
@@ -533,8 +523,6 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 
 		text_poke_early(instr, insn_buff, insn_buff_sz);
 	}
-
-	kasan_enable_current();
 }
 
 static inline bool is_jcc32(struct insn *insn)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ea49322ba151..e1f8a7de07cc 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1,6 +1,4 @@
 // SPDX-License-Identifier: GPL-2.0-only
-/* cpu_feature_enabled() cannot be used this early */
-#define USE_EARLY_PGTABLE_L5
 
 #include <linux/memblock.h>
 #include <linux/linkage.h>
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 14f7dda20954..84b5df539a94 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -5,9 +5,6 @@
  *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
  */
 
-/* cpu_feature_enabled() cannot be used this early */
-#define USE_EARLY_PGTABLE_L5
-
 #include <linux/init.h>
 #include <linux/linkage.h>
 #include <linux/types.h>
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 0539efd0d216..7c4fafbd52cc 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -1,9 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #define pr_fmt(fmt) "kasan: " fmt
 
-/* cpu_feature_enabled() cannot be used this early */
-#define USE_EARLY_PGTABLE_L5
-
 #include <linux/memblock.h>
 #include <linux/kasan.h>
 #include <linux/kdebug.h>
-- 
2.49.0.1045.g170613ef41-goog