[PATCH v3 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS

[Andrey Konovalov]

Currently, KASAN_SW_TAGS uses 0xFF as the default tag value for
unallocated memory. The underlying idea is that since that memory
hasn't been allocated yet, it's only supposed to be dereferenced
through a pointer with the native 0xFF tag.

While this is a good idea in terms on consistency, practically it
doesn't bring any benefit. Since the 0xFF pointer tag is a match-all
tag, it doesn't matter what tag the accessed memory has. No accesses
through 0xFF-tagged pointers are considered buggy by KASAN.

This patch changes the default tag value for unallocated memory to 0xFE,
which is the tag KASAN uses for inaccessible memory. This doesn't affect
accesses through 0xFF-tagged pointer to this memory, buut this allows
KASAN to detect wild and large out-of-bounds invalid memory accesses
through otherwise-tagged pointers.

This is a prepatory patch for the next one, which changes the tag-based
KASAN modes to not poison the boot memory.

Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
---
 include/linux/kasan.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index b91732bd05d7..1d89b8175027 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -30,7 +30,8 @@ struct kunit_kasan_expectation {
 /* Software KASAN implementations use shadow memory. */
 
 #ifdef CONFIG_KASAN_SW_TAGS
-#define KASAN_SHADOW_INIT 0xFF
+/* This matches KASAN_TAG_INVALID. */
+#define KASAN_SHADOW_INIT 0xFE
 #else
 #define KASAN_SHADOW_INIT 0
 #endif
-- 
2.30.1.766.gb4fecdf3b7-goog