mm/kasan: fix false positive invalid-free reports with CONFIG_KASAN_SW_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/20190819172540.19581-1-aryabinin@virtuozzo.com
Fixes: 7f94ffbc ("kasan: add hooks implementation for tag-based mode")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reported-by: Walter Wu <walter-zh.wu@mediatek.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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,