From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 817E426F288; Thu, 12 Mar 2026 20:25:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773347156; cv=none; b=lX1fU1hlnkNxfwRXxIT0moXwg+cP+iGFFDkwP7vMArSLDz/z9ryeo9BHuVBk8St1zti1UzBEt2kC0cjBszBcYLvuEUJv4rzajRewhyTRF8RbEdbsMoPUo9iQlKCFDIMszrJl/HC034BksexHFwbvrg0agVeMWQw8ZfVTXWeHd4M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773347156; c=relaxed/simple; bh=RCd4Svm9DS7wz9C2u8Vx0JlMamTHAEgsTdbHF9iBMkw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aIB4ggr1ujnCcOnAzFE+vqlJ/c6lJnwmwclF0wNBkkP5FJvozFjFn8/DuovCNpc0AwOpmlwxhuP9Pv7u43qpPmyD4la8J/MX+LoYuxlezsLzp6Tvf7nRPDVXQVIw4v2ADOkEepsAmucqgRMs0bzeSq657ogdpt+vQb7j2rNg8Jo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=IdKsz0kR; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="IdKsz0kR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C2639C4CEF7; Thu, 12 Mar 2026 20:25:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1773347156; bh=RCd4Svm9DS7wz9C2u8Vx0JlMamTHAEgsTdbHF9iBMkw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=IdKsz0kR+vYPAXGCY/QsTYHoiMJlUwndM4paQcgAH1k0hIWxurmhdXktg4k4Jtqi3 vvzLnJChA6nw0VAW2EHOc5xzNgTewHkEKW2thVXk5ndvgVHcIeVPtXu4GlABZk/NF3 /EdHzJpBKJgipJq/5G1ZizVRZPpgick6rwb9I5go= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Yung Chih Su , Eric Dumazet , Jakub Kicinski , Sasha Levin Subject: [PATCH 6.12 218/265] net: ipv4: fix ARM64 alignment fault in multipath hash seed Date: Thu, 12 Mar 2026 21:10:05 +0100 Message-ID: <20260312201026.197117813@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260312201018.128816016@linuxfoundation.org> References: <20260312201018.128816016@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Yung Chih Su [ Upstream commit 4ee7fa6cf78ff26d783d39e2949d14c4c1cd5e7f ] `struct sysctl_fib_multipath_hash_seed` contains two u32 fields (user_seed and mp_seed), making it an 8-byte structure with a 4-byte alignment requirement. In `fib_multipath_hash_from_keys()`, the code evaluates the entire struct atomically via `READ_ONCE()`: mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed; While this silently works on GCC by falling back to unaligned regular loads which the ARM64 kernel tolerates, it causes a fatal kernel panic when compiled with Clang and LTO enabled. Commit e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y") strengthens `READ_ONCE()` to use Load-Acquire instructions (`ldar` / `ldapr`) to prevent compiler reordering bugs under Clang LTO. Since the macro evaluates the full 8-byte struct, Clang emits a 64-bit `ldar` instruction. ARM64 architecture strictly requires `ldar` to be naturally aligned, thus executing it on a 4-byte aligned address triggers a strict Alignment Fault (FSC = 0x21). Fix the read side by moving the `READ_ONCE()` directly to the `u32` member, which emits a safe 32-bit `ldar Wn`. Furthermore, Eric Dumazet pointed out that `WRITE_ONCE()` on the entire struct in `proc_fib_multipath_hash_set_seed()` is also flawed. Analysis shows that Clang splits this 8-byte write into two separate 32-bit `str` instructions. While this avoids an alignment fault, it destroys atomicity and exposes a tear-write vulnerability. Fix this by explicitly splitting the write into two 32-bit `WRITE_ONCE()` operations. Finally, add the missing `READ_ONCE()` when reading `user_seed` in `proc_fib_multipath_hash_seed()` to ensure proper pairing and concurrency safety. Fixes: 4ee2a8cace3f ("net: ipv4: Add a sysctl to set multipath hash seed") Signed-off-by: Yung Chih Su Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260302060247.7066-1-yuuchihsu@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- include/net/ip_fib.h | 2 +- net/ipv4/sysctl_net_ipv4.c | 5 +++-- 2 files changed, 4 insertions(+), 3 deletions(-) diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h index 967e4dc555fac..339b92cd5cec6 100644 --- a/include/net/ip_fib.h +++ b/include/net/ip_fib.h @@ -544,7 +544,7 @@ static inline u32 fib_multipath_hash_from_keys(const struct net *net, siphash_aligned_key_t hash_key; u32 mp_seed; - mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed; + mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.mp_seed); fib_multipath_hash_construct_key(&hash_key, mp_seed); return flow_hash_from_keys_seed(keys, &hash_key); diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index a79b2a52ce01e..8d411cce0aedc 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -481,7 +481,8 @@ static void proc_fib_multipath_hash_set_seed(struct net *net, u32 user_seed) proc_fib_multipath_hash_rand_seed), }; - WRITE_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed, new); + WRITE_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.user_seed, new.user_seed); + WRITE_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.mp_seed, new.mp_seed); } static int proc_fib_multipath_hash_seed(const struct ctl_table *table, int write, @@ -495,7 +496,7 @@ static int proc_fib_multipath_hash_seed(const struct ctl_table *table, int write int ret; mphs = &net->ipv4.sysctl_fib_multipath_hash_seed; - user_seed = mphs->user_seed; + user_seed = READ_ONCE(mphs->user_seed); tmp = *table; tmp.data = &user_seed; -- 2.51.0