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From: Andrey Konovalov <>
To: Andrew Morton <>
Cc: Catalin Marinas <>,
	Will Deacon <>,
	 Vincenzo Frascino <>,
	Dmitry Vyukov <>,
	 Andrey Ryabinin <>,
	Alexander Potapenko <>,
	 Marco Elver <>,
	Peter Collingbourne <>,
	Evgenii Stepanov <>,
	 Branislav Rankov <>,
	Kevin Brodsky <>,
	 Christoph Hellwig <>,,,,,
	Andrey Konovalov <>
Subject: [PATCH v2 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS
Date: Fri, 19 Feb 2021 01:22:23 +0100	[thread overview]
Message-ID: <> (raw)

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, but 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 <>
 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 14f72ec96492..44c147dae7e3 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. */
+/* This matches KASAN_TAG_INVALID. */

             reply	other threads:[~2021-02-19  0:22 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19  0:22 Andrey Konovalov [this message]
2021-02-19  0:22 ` [PATCH v2 2/2] mm, kasan: don't poison boot memory with tag-based modes Andrey Konovalov
2021-03-05 15:46 ` [PATCH v2 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS Andrey Konovalov

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