[PATCH 01/10] 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>
Signed-off-by: Ethan Graham <ethan.w.s.graham@gmail.com>
Reviewed-by: Alexander Potapenko <glider@google.com>

---
PR v3:
- Move kasan_poison_range into mm/kasan/common.c so that it is built
  with HW_TAGS mode enabled.
- Add a runtime check for kasan_enabled() in kasan_poison_range.
- Add two WARN_ON()s in kasan_poison_range when the input is invalid.
PR v1:
- Enforce KASAN_GRANULE_SIZE alignment for the end of the range in
  kasan_poison_range(), and return -EINVAL when this isn't respected.
---
---
 include/linux/kasan.h | 11 +++++++++++
 mm/kasan/common.c     | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2b..cd6cdf732378 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -102,6 +102,16 @@ static inline bool kasan_has_integrated_init(void)
 }
 
 #ifdef CONFIG_KASAN
+
+/**
+ * kasan_poison_range - poison the memory range [@addr, @addr + @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=y.
+ */
+int kasan_poison_range(const void *addr, 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 +412,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
 
 #else /* CONFIG_KASAN */
 
+static inline int kasan_poison_range(const void *start, size_t size) { return 0; }
 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/common.c b/mm/kasan/common.c
index 9142964ab9c9..c83579ef37c6 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -570,3 +570,40 @@ bool __kasan_check_byte(const void *address, unsigned long ip)
 	}
 	return true;
 }
+
+int kasan_poison_range(const void *addr, size_t size)
+{
+	uintptr_t start_addr = (uintptr_t)addr;
+	uintptr_t head_granule_start;
+	uintptr_t poison_body_start;
+	uintptr_t poison_body_end;
+	size_t head_prefix_size;
+	uintptr_t end_addr;
+
+	if (!kasan_enabled())
+		return 0;
+
+	end_addr = start_addr + size;
+	if (WARN_ON(end_addr % KASAN_GRANULE_SIZE))
+		return -EINVAL;
+
+	if (WARN_ON(start_addr >= end_addr))
+		return -EINVAL;
+
+	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 = end_addr;
+
+	if (poison_body_start < poison_body_end)
+		kasan_poison((void *)poison_body_start,
+			     poison_body_end - poison_body_start,
+			     KASAN_SLAB_REDZONE, false);
+	return 0;
+}
+EXPORT_SYMBOL(kasan_poison_range);
-- 
2.51.0