Commit d94c12bd authored by Peter Collingbourne's avatar Peter Collingbourne Committed by Kees Cook
Browse files

string: Add load_unaligned_zeropad() code path to sized_strscpy() 



The call to read_word_at_a_time() in sized_strscpy() is problematic
with MTE because it may trigger a tag check fault when reading
across a tag granule (16 bytes) boundary. To make this code
MTE compatible, let's start using load_unaligned_zeropad()
on architectures where it is available (i.e. architectures that
define CONFIG_DCACHE_WORD_ACCESS). Because load_unaligned_zeropad()
takes care of page boundaries as well as tag granule boundaries,
also disable the code preventing crossing page boundaries when using
load_unaligned_zeropad().

Signed-off-by: default avatarPeter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/If4b22e43b5a4ca49726b4bf98ada827fdf755548


Fixes: 94ab5b61 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
Cc: stable@vger.kernel.org
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250403000703.2584581-2-pcc@google.com


Signed-off-by: default avatarKees Cook <kees@kernel.org>
parent 8ffd015d

lib/string.c

+10 −3
Original line number Diff line number Diff line
@@ -119,6 +119,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) 
	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))

		return -E2BIG;



#ifndef CONFIG_DCACHE_WORD_ACCESS

#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS

	/*

	 * If src is unaligned, don't cross a page boundary,
@@ -133,12 +134,14 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) 
	/* If src or dest is unaligned, don't do word-at-a-time. */

	if (((long) dest | (long) src) & (sizeof(long) - 1))

		max = 0;

#endif

#endif



	/*

	 * read_word_at_a_time() below may read uninitialized bytes after the

	 * trailing zero and use them in comparisons. Disable this optimization

	 * under KMSAN to prevent false positive reports.

	 * load_unaligned_zeropad() or read_word_at_a_time() below may read

	 * uninitialized bytes after the trailing zero and use them in

	 * comparisons. Disable this optimization under KMSAN to prevent

	 * false positive reports.

	 */

	if (IS_ENABLED(CONFIG_KMSAN))

		max = 0;
@@ -146,7 +149,11 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) 
	while (max >= sizeof(unsigned long)) {

		unsigned long c, data;



#ifdef CONFIG_DCACHE_WORD_ACCESS

		c = load_unaligned_zeropad(src+res);

#else

		c = read_word_at_a_time(src+res);

#endif

		if (has_zero(c, &data, &constants)) {

			data = prep_zero_mask(c, data, &constants);

			data = create_zero_mask(data);