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

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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: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/If4b22e43b5a4ca49726b4bf98ada827fdf755548
Fixes: 94ab5b61ee ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
Cc: stable@vger.kernel.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250403000703.2584581-2-pcc@google.com
Signed-off-by: Kees Cook <kees@kernel.org>
This commit is contained in:
Peter Collingbourne 2025-04-02 17:06:59 -07:00 • committed by Kees Cook
parent 8ffd015db8
commit d94c12bd97
1 changed files with 10 additions and 3 deletions
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13
lib/string.c
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@ -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);