[PATCH v2] string: copy byte by byte in sized_strscpy() under KASAN

[Qiliang Yuan <odys.yuan@gmail.com>]

sized_strscpy() copies a word at a time. The loads go through
read_word_at_a_time(), which checks the first byte of each word with
KASAN, or through load_unaligned_zeropad() on architectures with
CONFIG_DCACHE_WORD_ACCESS. Under KMSAN it copies byte by byte instead.

load_unaligned_zeropad() is inline assembly that the compiler does not
instrument. Since sized_strscpy() switched to it, generic and software
tag-based KASAN no longer report strscpy() reading from a freed or
out-of-bounds source on x86_64, arm64 and the other architectures that
select CONFIG_DCACHE_WORD_ACCESS. On x86_64 the kasan_strings KUnit
case, which expects a report when the first source byte is out of
bounds, fails as a result:

  KASAN and string KUnit suites      failures
  on x86_64, generic KASAN        before    after
  CONFIG_FORTIFY_SOURCE=n              1        0
  CONFIG_FORTIFY_SOURCE=y              1        0

Copy byte by byte under generic and software tag-based KASAN too, as
under KMSAN, so that every byte strscpy() reads is checked. Hardware
tag-based KASAN keeps the word loop that load_unaligned_zeropad() was
brought in for.

Fixes: d94c12bd97d5 ("string: Add load_unaligned_zeropad() code path to sized_strscpy()")
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
V1 -> V2:
- Copy byte by byte under generic and software tag-based KASAN instead
  of checking the first byte of each word (Andrew Morton)
- Rebase onto v7.3

v1: https://lore.kernel.org/r/20261018-bug-string-strscpy-kasan-first-byte-v1-1-375a2c46d88f@gmail.com

The other callers of load_unaligned_zeropad() are covered by a separate
series (Peter Collingbourne):
https://lore.kernel.org/r/20261019-bug-word-at-a-time-zeropad-kasan-v1-0-f21a3ed6a477@gmail.com
---
 lib/string.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/string.c b/lib/string.c
index a3778d5aab4af..d14e42ae3903b 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -124,8 +124,14 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
 	 * uninitialized bytes after the trailing zero and use them in
 	 * comparisons. Disable this optimization under KMSAN to prevent
 	 * false positive reports.
+	 *
+	 * KASAN does not see load_unaligned_zeropad() at all, and
+	 * read_word_at_a_time() only checks the first byte of each word.
+	 * Disable it under software KASAN as well, so that every byte read
+	 * is checked.
 	 */
-	if (IS_ENABLED(CONFIG_KMSAN))
+	if (IS_ENABLED(CONFIG_KMSAN) || IS_ENABLED(CONFIG_KASAN_GENERIC) ||
+	    IS_ENABLED(CONFIG_KASAN_SW_TAGS))
 		max = 0;
 
 	while (max >= sizeof(unsigned long)) {

---
base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
change-id: 20261016-bug-string-strscpy-kasan-first-byte-335585ce203d

Best regards,
-- 
Qiliang Yuan <odys.yuan@gmail.com>