[PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range

[Ethan Graham <ethan.w.s.graham@gmail.com>]

From: Ethan Graham <ethangraham@google.com>

Introduce a new helper function, kasan_poison_range(), to encapsulate
the logic for poisoning an arbitrary memory range of a given size, and
expose it publically in <include/linux/kasan.h>.

This is a preparatory change for the upcoming KFuzzTest patches, which
requires the ability to poison the inter-region padding in its input
buffers.

No functional change to any other subsystem is intended by this commit.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 include/linux/kasan.h | 16 ++++++++++++++++
 mm/kasan/shadow.c     | 31 +++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2b..09baeb6c9f4d 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -102,6 +102,21 @@ static inline bool kasan_has_integrated_init(void)
 }
 
 #ifdef CONFIG_KASAN
+
+/**
+ * kasan_poison_range - poison the memory range [start, start + size)
+ *
+ * The exact behavior is subject to alignment with KASAN_GRANULE_SIZE, defined
+ * in <mm/kasan/kasan.h>.
+ *
+ * - If @start is unaligned, the initial partial granule at the beginning
+ *	of the range is only poisoned if CONFIG_KASAN_GENERIC is enabled.
+ * - The poisoning of the range only extends up to the last full granule before
+ *	the end of the range. Any remaining bytes in a final partial granule are
+ *	ignored.
+ */
+void kasan_poison_range(const void *start, size_t size);
+
 void __kasan_unpoison_range(const void *addr, size_t size);
 static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
 {
@@ -402,6 +417,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
 
 #else /* CONFIG_KASAN */
 
+static inline void kasan_poison_range(const void *start, size_t size) {}
 static inline void kasan_unpoison_range(const void *address, size_t size) {}
 static inline void kasan_poison_pages(struct page *page, unsigned int order,
 				      bool init) {}
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d2c70cd2afb1..a1b6bfb35f07 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -147,6 +147,37 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
 }
 EXPORT_SYMBOL_GPL(kasan_poison);
 
+void kasan_poison_range(const void *start, size_t size)
+{
+	void *end = (char *)start + size;
+	uintptr_t start_addr = (uintptr_t)start;
+	uintptr_t head_granule_start;
+	uintptr_t poison_body_start;
+	uintptr_t poison_body_end;
+	size_t head_prefix_size;
+	uintptr_t end_addr;
+
+	end_addr = ALIGN_DOWN((uintptr_t)end, KASAN_GRANULE_SIZE);
+	if (start_addr >= end_addr)
+		return;
+
+	head_granule_start = ALIGN_DOWN(start_addr, KASAN_GRANULE_SIZE);
+	head_prefix_size = start_addr - head_granule_start;
+
+	if (IS_ENABLED(CONFIG_KASAN_GENERIC) && head_prefix_size > 0)
+		kasan_poison_last_granule((void *)head_granule_start,
+					  head_prefix_size);
+
+	poison_body_start = ALIGN(start_addr, KASAN_GRANULE_SIZE);
+	poison_body_end = ALIGN_DOWN(end_addr, KASAN_GRANULE_SIZE);
+
+	if (poison_body_start < poison_body_end)
+		kasan_poison((void *)poison_body_start,
+			     poison_body_end - poison_body_start,
+			     KASAN_SLAB_REDZONE, false);
+}
+EXPORT_SYMBOL(kasan_poison_range);
+
 #ifdef CONFIG_KASAN_GENERIC
 void kasan_poison_last_granule(const void *addr, size_t size)
 {
-- 
2.51.0.318.gd7df087d1a-goog