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

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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: ece69af2ed ("rwonce: handle KCSAN like KASAN in read_word_at_a_time()")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/r/20250326203926.GA10484@ax162
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
This commit is contained in:
Jann Horn 2025-03-26 22:04:36 +01:00 committed by Arnd Bergmann
parent ece69af2ed
commit 47a60391ae
1 changed files with 6 additions and 1 deletions
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7
include/asm-generic/rwonce.h
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@ -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__ */