[PATCH] mm/kasan: Fix false positive invalid-free reports with CONFIG_KASAN_SW_TAGS=y

[Andrey Ryabinin]

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 loose 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.

Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Reported-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 mm/kasan/common.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 895dc5e2b3d5..3b8cde0cb5b2 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -406,8 +406,14 @@ 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,
-- 
2.21.0