In the Linux kernel, the following vulnerability has been resolved:
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.