+ kasan-arm64-improve-comments-for-kasan_shadow_start-end.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: kasan/arm64: improve comments for KASAN_SHADOW_START/END
has been added to the -mm mm-unstable branch.  Its filename is
     kasan-arm64-improve-comments-for-kasan_shadow_start-end.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kasan-arm64-improve-comments-for-kasan_shadow_start-end.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Andrey Konovalov <andreyknvl@google.com>
Subject: kasan/arm64: improve comments for KASAN_SHADOW_START/END
Date: Thu, 21 Dec 2023 21:04:43 +0100

Patch series "kasan: assorted clean-ups".

Code clean-ups, nothing worthy of being backported to stable.


This patch (of 11):

Unify and improve the comments for KASAN_SHADOW_START/END definitions from
include/asm/kasan.h and include/asm/memory.h.

Also put both definitions together in include/asm/memory.h.

Also clarify the related BUILD_BUG_ON checks in mm/kasan_init.c.

Link: https://lkml.kernel.org/r/cover.1703188911.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/140108ca0b164648c395a41fbeecb0601b1ae9e1.1703188911.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 arch/arm64/include/asm/kasan.h  |   22 -----------------
 arch/arm64/include/asm/memory.h |   38 +++++++++++++++++++++++++-----
 arch/arm64/mm/kasan_init.c      |    5 +++
 3 files changed, 38 insertions(+), 27 deletions(-)

--- a/arch/arm64/include/asm/kasan.h~kasan-arm64-improve-comments-for-kasan_shadow_start-end
+++ a/arch/arm64/include/asm/kasan.h
@@ -15,29 +15,9 @@
 
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 
+asmlinkage void kasan_early_init(void);
 void kasan_init(void);
-
-/*
- * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
- * KASAN_SHADOW_END: KASAN_SHADOW_START + 1/N of kernel virtual addresses,
- * where N = (1 << KASAN_SHADOW_SCALE_SHIFT).
- *
- * KASAN_SHADOW_OFFSET:
- * This value is used to map an address to the corresponding shadow
- * address by the following formula:
- *     shadow_addr = (address >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET
- *
- * (1 << (64 - KASAN_SHADOW_SCALE_SHIFT)) shadow addresses that lie in range
- * [KASAN_SHADOW_OFFSET, KASAN_SHADOW_END) cover all 64-bits of virtual
- * addresses. So KASAN_SHADOW_OFFSET should satisfy the following equation:
- *      KASAN_SHADOW_OFFSET = KASAN_SHADOW_END -
- *				(1ULL << (64 - KASAN_SHADOW_SCALE_SHIFT))
- */
-#define _KASAN_SHADOW_START(va)	(KASAN_SHADOW_END - (1UL << ((va) - KASAN_SHADOW_SCALE_SHIFT)))
-#define KASAN_SHADOW_START      _KASAN_SHADOW_START(vabits_actual)
-
 void kasan_copy_shadow(pgd_t *pgdir);
-asmlinkage void kasan_early_init(void);
 
 #else
 static inline void kasan_init(void) { }
--- a/arch/arm64/include/asm/memory.h~kasan-arm64-improve-comments-for-kasan_shadow_start-end
+++ a/arch/arm64/include/asm/memory.h
@@ -65,15 +65,41 @@
 #define KERNEL_END		_end
 
 /*
- * Generic and tag-based KASAN require 1/8th and 1/16th of the kernel virtual
- * address space for the shadow region respectively. They can bloat the stack
- * significantly, so double the (minimum) stack size when they are in use.
+ * Generic and Software Tag-Based KASAN modes require 1/8th and 1/16th of the
+ * kernel virtual address space for storing the shadow memory respectively.
+ *
+ * The mapping between a virtual memory address and its corresponding shadow
+ * memory address is defined based on the formula:
+ *
+ *     shadow_addr = (addr >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET
+ *
+ * where KASAN_SHADOW_SCALE_SHIFT is the order of the number of bits that map
+ * to a single shadow byte and KASAN_SHADOW_OFFSET is a constant that offsets
+ * the mapping. Note that KASAN_SHADOW_OFFSET does not point to the start of
+ * the shadow memory region.
+ *
+ * Based on this mapping, we define two constants:
+ *
+ *     KASAN_SHADOW_START: the start of the shadow memory region;
+ *     KASAN_SHADOW_END: the end of the shadow memory region.
+ *
+ * KASAN_SHADOW_END is defined first as the shadow address that corresponds to
+ * the upper bound of possible virtual kernel memory addresses UL(1) << 64
+ * according to the mapping formula.
+ *
+ * KASAN_SHADOW_START is defined second based on KASAN_SHADOW_END. The shadow
+ * memory start must map to the lowest possible kernel virtual memory address
+ * and thus it depends on the actual bitness of the address space.
+ *
+ * As KASAN inserts redzones between stack variables, this increases the stack
+ * memory usage significantly. Thus, we double the (minimum) stack size.
  */
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 #define KASAN_SHADOW_OFFSET	_AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
-#define KASAN_SHADOW_END	((UL(1) << (64 - KASAN_SHADOW_SCALE_SHIFT)) \
-					+ KASAN_SHADOW_OFFSET)
-#define PAGE_END		(KASAN_SHADOW_END - (1UL << (vabits_actual - KASAN_SHADOW_SCALE_SHIFT)))
+#define KASAN_SHADOW_END	((UL(1) << (64 - KASAN_SHADOW_SCALE_SHIFT)) + KASAN_SHADOW_OFFSET)
+#define _KASAN_SHADOW_START(va)	(KASAN_SHADOW_END - (UL(1) << ((va) - KASAN_SHADOW_SCALE_SHIFT)))
+#define KASAN_SHADOW_START	_KASAN_SHADOW_START(vabits_actual)
+#define PAGE_END		KASAN_SHADOW_START
 #define KASAN_THREAD_SHIFT	1
 #else
 #define KASAN_THREAD_SHIFT	0
--- a/arch/arm64/mm/kasan_init.c~kasan-arm64-improve-comments-for-kasan_shadow_start-end
+++ a/arch/arm64/mm/kasan_init.c
@@ -170,6 +170,11 @@ asmlinkage void __init kasan_early_init(
 {
 	BUILD_BUG_ON(KASAN_SHADOW_OFFSET !=
 		KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT)));
+	/*
+	 * We cannot check the actual value of KASAN_SHADOW_START during build,
+	 * as it depends on vabits_actual. As a best-effort approach, check
+	 * potential values calculated based on VA_BITS and VA_BITS_MIN.
+	 */
 	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS), PGDIR_SIZE));
 	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS_MIN), PGDIR_SIZE));
 	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
_

Patches currently in -mm which might be from andreyknvl@google.com are

kasan-rename-kasan_slab_free_mempool-to-kasan_mempool_poison_object.patch
kasan-move-kasan_mempool_poison_object.patch
kasan-document-kasan_mempool_poison_object.patch
kasan-add-return-value-for-kasan_mempool_poison_object.patch
kasan-introduce-kasan_mempool_unpoison_object.patch
kasan-introduce-kasan_mempool_poison_pages.patch
kasan-introduce-kasan_mempool_unpoison_pages.patch
kasan-clean-up-__kasan_mempool_poison_object.patch
kasan-save-free-stack-traces-for-slab-mempools.patch
kasan-clean-up-and-rename-____kasan_kmalloc.patch
kasan-introduce-poison_kmalloc_large_redzone.patch
kasan-save-alloc-stack-traces-for-mempool.patch
mempool-skip-slub_debug-poisoning-when-kasan-is-enabled.patch
mempool-use-new-mempool-kasan-hooks.patch
mempool-introduce-mempool_use_prealloc_only.patch
kasan-add-mempool-tests.patch
kasan-rename-pagealloc-tests.patch
kasan-reorder-tests.patch
kasan-rename-and-document-kasan_unpoison_object_data.patch
skbuff-use-mempool-kasan-hooks.patch
io_uring-use-mempool-kasan-hook.patch
lib-stackdepot-add-printk_deferred_enter-exit-guards.patch
kasan-handle-concurrent-kasan_record_aux_stack-calls.patch
kasan-memset-free-track-in-qlink_free.patch
lib-stackdepot-fix-comment-in-include-linux-stackdepoth.patch
kasan-arm64-improve-comments-for-kasan_shadow_start-end.patch
mm-kasan-use-kasan_tag_kernel-instead-of-0xff.patch
kasan-improve-kasan_non_canonical_hook.patch
kasan-clean-up-kasan_requires_meta.patch
kasan-update-kasan_poison-documentation-comment.patch
kasan-clean-up-is_kfence_address-checks.patch
kasan-respect-config_kasan_vmalloc-for-kasan_flag_vmalloc.patch
kasan-check-kasan_vmalloc_enabled-in-vmalloc-tests.patch
kasan-export-kasan_poison-as-gpl.patch
kasan-remove-slub-checks-for-page_alloc-fallbacks-in-tests.patch
kasan-speed-up-match_all_mem_tag-test-for-sw_tags.patch