[PATCH v5 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy()

[PATCH v5 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy()

[Peter Collingbourne]

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: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
Cc: stable@vger.kernel.org
---
v2:
- new approach

 lib/string.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index eb4486ed40d25..b632c71df1a50 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -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);
-- 
2.49.0.472.ge94155a9ec-goog

Re: [PATCH v5 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy()

[Catalin Marinas]

On Wed, Apr 02, 2025 at 05:06:59PM -0700, Peter Collingbourne wrote:
> 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: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
> Cc: stable@vger.kernel.org

Up to you if you want to keep the panic behaviour on unmapped pages.
Either way:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>