kasan: fix bug detection via ksize for HW_TAGS mode

The currently existing kasan_check_read/write() annotations are intended
to be used for kernel modules that have KASAN compiler instrumentation
disabled. Thus, they are only relevant for the software KASAN modes that
rely on compiler instrumentation.

However there's another use case for these annotations: ksize() checks
that the object passed to it is indeed accessible before unpoisoning the
whole object. This is currently done via __kasan_check_read(), which is
compiled away for the hardware tag-based mode that doesn't rely on
compiler instrumentation. This leads to KASAN missing detecting some
memory corruptions.

Provide another annotation called kasan_check_byte() that is available
for all KASAN modes. As the implementation rename and reuse
kasan_check_invalid_free(). Use this new annotation in ksize().
To avoid having ksize() as the top frame in the reported stack trace
pass _RET_IP_ to __kasan_check_byte().

Also add a new ksize_uaf() test that checks that a use-after-free is
detected via ksize() itself, and via plain accesses that happen later.

Link: https://linux-review.googlesource.com/id/Iaabf771881d0f9ce1b969f2a62938e99d3308ec5
Link: https://lkml.kernel.org/r/f32ad74a60b28d8402482a38476f02bb7600f620.1610733117.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Reviewed-by: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Branislav Rankov <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Evgenii Stepanov <[email protected]>
Cc: Kevin Brodsky <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/kasan-checks.h

@@ -4,6 +4,12 @@
 
 #include <linux/types.h>
 
+/*
+ * The annotations present in this file are only relevant for the software
+ * KASAN modes that rely on compiler instrumentation, and will be optimized
+ * away for the hardware tag-based KASAN mode. Use kasan_check_byte() instead.
+ */
+
 /*
  * __kasan_check_*: Always available when KASAN is enabled. This may be used
  * even in compilation units that selectively disable KASAN, but must use KASAN

include/linux/kasan.h

@@ -246,6 +246,19 @@ static __always_inline void kasan_kfree_large(void *ptr)
 		__kasan_kfree_large(ptr, _RET_IP_);
 }
 
+/*
+ * Unlike kasan_check_read/write(), kasan_check_byte() is performed even for
+ * the hardware tag-based mode that doesn't rely on compiler instrumentation.
+ */
+bool __kasan_check_byte(const void *addr, unsigned long ip);
+static __always_inline bool kasan_check_byte(const void *addr)
+{
+	if (kasan_enabled())
+		return __kasan_check_byte(addr, _RET_IP_);
+	return true;
+}
+
+
 bool kasan_save_enable_multi_shot(void);
 void kasan_restore_multi_shot(bool enabled);
 
@@ -301,6 +314,10 @@ static inline void *kasan_krealloc(const void *object, size_t new_size,
 	return (void *)object;
 }
 static inline void kasan_kfree_large(void *ptr) {}
+static inline bool kasan_check_byte(const void *address)
+{
+	return true;
+}
 
 #endif /* CONFIG_KASAN */
 

lib/test_kasan.c

@@ -496,6 +496,7 @@ static void kasan_global_oob(struct kunit *test)
 	KUNIT_EXPECT_KASAN_FAIL(test, *(volatile char *)p);
 }
 
+/* Check that ksize() makes the whole object accessible. */
 static void ksize_unpoisons_memory(struct kunit *test)
 {
 	char *ptr;
@@ -514,6 +515,24 @@ static void ksize_unpoisons_memory(struct kunit *test)
 	kfree(ptr);
 }
 
+/*
+ * Check that a use-after-free is detected by ksize() and via normal accesses
+ * after it.
+ */
+static void ksize_uaf(struct kunit *test)
+{
+	char *ptr;
+	int size = 128 - KASAN_GRANULE_SIZE;
+
+	ptr = kmalloc(size, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
+	kfree(ptr);
+
+	KUNIT_EXPECT_KASAN_FAIL(test, ksize(ptr));
+	KUNIT_EXPECT_KASAN_FAIL(test, kasan_int_result = *ptr);
+	KUNIT_EXPECT_KASAN_FAIL(test, kasan_int_result = *(ptr + size));
+}
+
 static void kasan_stack_oob(struct kunit *test)
 {
 	char stack_array[10];
@@ -907,6 +926,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(kasan_alloca_oob_left),
 	KUNIT_CASE(kasan_alloca_oob_right),
 	KUNIT_CASE(ksize_unpoisons_memory),
+	KUNIT_CASE(ksize_uaf),
 	KUNIT_CASE(kmem_cache_double_free),
 	KUNIT_CASE(kmem_cache_invalid_free),
 	KUNIT_CASE(kasan_memchr),

mm/kasan/common.c

@@ -345,7 +345,7 @@ static bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
 	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
 		return false;
 
-	if (kasan_check_invalid_free(tagged_object)) {
+	if (!kasan_byte_accessible(tagged_object)) {
 		kasan_report_invalid_free(tagged_object, ip);
 		return true;
 	}
@@ -490,3 +490,12 @@ void __kasan_kfree_large(void *ptr, unsigned long ip)
 		kasan_report_invalid_free(ptr, ip);
 	/* The object will be poisoned by kasan_free_pages(). */
 }
+
+bool __kasan_check_byte(const void *address, unsigned long ip)
+{
+	if (!kasan_byte_accessible(address)) {
+		kasan_report((unsigned long)address, 1, false, ip);
+		return false;
+	}
+	return true;
+}

mm/kasan/generic.c

@@ -185,11 +185,11 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
 	return check_region_inline(addr, size, write, ret_ip);
 }
 
-bool kasan_check_invalid_free(void *addr)
+bool kasan_byte_accessible(const void *addr)
 {
 	s8 shadow_byte = READ_ONCE(*(s8 *)kasan_mem_to_shadow(addr));
 
-	return shadow_byte < 0 || shadow_byte >= KASAN_GRANULE_SIZE;
+	return shadow_byte >= 0 && shadow_byte < KASAN_GRANULE_SIZE;
 }
 
 void kasan_cache_shrink(struct kmem_cache *cache)

mm/kasan/kasan.h

@@ -341,20 +341,20 @@ static inline void kasan_unpoison(const void *address, size_t size)
 			round_up(size, KASAN_GRANULE_SIZE), get_tag(address));
 }
 
-static inline bool kasan_check_invalid_free(void *addr)
+static inline bool kasan_byte_accessible(const void *addr)
 {
 	u8 ptr_tag = get_tag(addr);
-	u8 mem_tag = hw_get_mem_tag(addr);
+	u8 mem_tag = hw_get_mem_tag((void *)addr);
 
-	return (mem_tag == KASAN_TAG_INVALID) ||
-		(ptr_tag != KASAN_TAG_KERNEL && ptr_tag != mem_tag);
+	return (mem_tag != KASAN_TAG_INVALID) &&
+		(ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
 }
 
 #else /* CONFIG_KASAN_HW_TAGS */
 
 void kasan_poison(const void *address, size_t size, u8 value);
 void kasan_unpoison(const void *address, size_t size);
-bool kasan_check_invalid_free(void *addr);
+bool kasan_byte_accessible(const void *addr);
 
 #endif /* CONFIG_KASAN_HW_TAGS */
 

mm/kasan/sw_tags.c

@@ -118,13 +118,13 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
 	return true;
 }
 
-bool kasan_check_invalid_free(void *addr)
+bool kasan_byte_accessible(const void *addr)
 {
 	u8 tag = get_tag(addr);
 	u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
 
-	return (shadow_byte == KASAN_TAG_INVALID) ||
-		(tag != KASAN_TAG_KERNEL && tag != shadow_byte);
+	return (shadow_byte != KASAN_TAG_INVALID) &&
+		(tag == KASAN_TAG_KERNEL || tag == shadow_byte);
 }
 
 #define DEFINE_HWASAN_LOAD_STORE(size)					\

mm/slab_common.c

@@ -1218,19 +1218,21 @@ size_t ksize(const void *objp)
 	size_t size;
 
 	/*
-	 * We need to check that the pointed to object is valid, and only then
-	 * unpoison the shadow memory below. We use __kasan_check_read(), to
-	 * generate a more useful report at the time ksize() is called (rather
-	 * than later where behaviour is undefined due to potential
-	 * use-after-free or double-free).
+	 * We need to first check that the pointer to the object is valid, and
+	 * only then unpoison the memory. The report printed from ksize() is
+	 * more useful, then when it's printed later when the behaviour could
+	 * be undefined due to a potential use-after-free or double-free.
 	 *
-	 * If the pointed to memory is invalid we return 0, to avoid users of
+	 * We use kasan_check_byte(), which is supported for the hardware
+	 * tag-based KASAN mode, unlike kasan_check_read/write().
+	 *
+	 * If the pointed to memory is invalid, we return 0 to avoid users of
 	 * ksize() writing to and potentially corrupting the memory region.
 	 *
 	 * We want to perform the check before __ksize(), to avoid potentially
 	 * crashing in __ksize() due to accessing invalid metadata.
 	 */
-	if (unlikely(ZERO_OR_NULL_PTR(objp)) || !__kasan_check_read(objp, 1))
+	if (unlikely(ZERO_OR_NULL_PTR(objp)) || !kasan_check_byte(objp))
 		return 0;
 
 	size = __ksize(objp);