[PATCH] rwonce: fix crash by removing READ_ONCE() for unaligned read

[Jann Horn]

When arm64 is built with LTO, it upgrades READ_ONCE() to ldar / ldapr
(load-acquire) to avoid issues that can be caused by the compiler
optimizing away implicit address dependencies.

Unlike plain loads, these load-acquire instructions actually require an
aligned address.

For now, fix it by removing the READ_ONCE() that the buggy commit
introduced.

Fixes: ece69af2ede1 ("rwonce: handle KCSAN like KASAN in read_word_at_a_time()")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Closes: https://lore.kernel.org/r/20250326203926.GA10484@ax162
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 include/asm-generic/rwonce.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
index e9f2b84d2338..52b969c7cef9 100644
--- a/include/asm-generic/rwonce.h
+++ b/include/asm-generic/rwonce.h
@@ -86,7 +86,12 @@ unsigned long read_word_at_a_time(const void *addr)
 	kasan_check_read(addr, 1);
 	kcsan_check_read(addr, 1);
 
-	return READ_ONCE(*(unsigned long *)addr);
+	/*
+	 * This load can race with concurrent stores to out-of-bounds memory,
+	 * but READ_ONCE() can't be used because it requires higher alignment
+	 * than plain loads in arm64 builds with LTO.
+	 */
+	return *(unsigned long *)addr;
 }
 
 #endif /* __ASSEMBLY__ */

---
base-commit: ece69af2ede103e190ffdfccd9f9ec850606ab5e
change-id: 20250326-rwaat-fix-63d7557b3d88

-- 
Jann Horn <jannh@xxxxxxxxxx>