From 4ee7fa6cf78ff26d783d39e2949d14c4c1cd5e7f Mon Sep 17 00:00:00 2001 From: Yung Chih Su Date: Mon, 2 Mar 2026 14:02:47 +0800 Subject: net: ipv4: fix ARM64 alignment fault in multipath hash seed `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 --- include/net/ip_fib.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include') diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h index b4495c38e0a0..318593743b6e 100644 --- a/include/net/ip_fib.h +++ b/include/net/ip_fib.h @@ -559,7 +559,7 @@ 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); -- cgit v1.2.3