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Wed, 26 Mar 2025 17:24:03 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: T4094621b06357a4d Date: Wed, 26 Mar 2025 22:23:33 +0100 From: "Arnd Bergmann" To: "Jann Horn" Cc: "Marco Elver" , "Nathan Chancellor" , Linux-Arch , linux-kernel@vger.kernel.org, "Linus Torvalds" Message-Id: <4b412238-b20a-4346-bf67-f31df0a9f259@app.fastmail.com> In-Reply-To: <20250326-rwaat-fix-v1-1-600f411eaf23@google.com> References: <20250326-rwaat-fix-v1-1-600f411eaf23@google.com> Subject: Re: [PATCH] rwonce: fix crash by removing READ_ONCE() for unaligned read Content-Type: text/plain Content-Transfer-Encoding: 7bit On Wed, Mar 26, 2025, at 22:04, Jann Horn wrote: > 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 > Closes: https://lore.kernel.org/r/20250326203926.GA10484@ax162 > Signed-off-by: Jann Horn Thanks for the quick fix! I've applied this on top of the asm-generic branch, but I just sent the pull request with the regression to Linus an hour ago. I'll try to get a new pull request out tomorrow. Arnd > --- > 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