[PATCH 6.18 269/314] net: ipv4: fix ARM64 alignment fault in multipath hash seed

Sasha Levin
2026-03-10T11:29:40Z

From: Yung Chih Su <yuuchihsu@gmail.com>

[ 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 <yuuchihsu@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260302060247.7066-1-yuuchihsu@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 b4495c38e0a01..318593743b6e1 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);
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 24dbc603cc44d..0f1dd75dbf37b 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -484,7 +484,8 @@ 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,
@@ -498,7 +499,7 @@ 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