mm/kasan: fix false positive invalid-free reports with CONFIG_KASAN_S…

…W_TAGS=y

The code like this:

	ptr = kmalloc(size, GFP_KERNEL);
	page = virt_to_page(ptr);
	offset = offset_in_page(ptr);
	kfree(page_address(page) + offset);

may produce false-positive invalid-free reports on the kernel with
CONFIG_KASAN_SW_TAGS=y.

In the example above we lose the original tag assigned to 'ptr', so
kfree() gets the pointer with 0xFF tag.  In kfree() we check that 0xFF
tag is different from the tag in shadow hence print false report.

Instead of just comparing tags, do the following:

1) Check that shadow doesn't contain KASAN_TAG_INVALID.  Otherwise it's
   double-free and it doesn't matter what tag the pointer have.

2) If pointer tag is different from 0xFF, make sure that tag in the
   shadow is the same as in the pointer.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 7f94ffb ("kasan: add hooks implementation for tag-based mode")
Signed-off-by: Andrey Ryabinin <[email protected]>
Reported-by: Walter Wu <[email protected]>
Reported-by: Mark Rutland <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kasan/common.c

@@ -407,8 +407,14 @@ static inline bool shadow_invalid(u8 tag, s8 shadow_byte)
 	if (IS_ENABLED(CONFIG_KASAN_GENERIC))
 		return shadow_byte < 0 ||
 			shadow_byte >= KASAN_SHADOW_SCALE_SIZE;
-	else
-		return tag != (u8)shadow_byte;
+
+	/* else CONFIG_KASAN_SW_TAGS: */
+	if ((u8)shadow_byte == KASAN_TAG_INVALID)
+		return true;
+	if ((tag != KASAN_TAG_KERNEL) && (tag != (u8)shadow_byte))
+		return true;
+
+	return false;
 }
 
 static bool __kasan_slab_free(struct kmem_cache *cache, void *object,