net: ipv4: fix ARM64 alignment fault in multipath hash seed

Overview

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.

Affected

• Linux >= 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 < 4bdc94d45d5459f0149085dfc1efe733c8e14f11
• Linux >= 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 < 7e4ad34a8889a6a9e0f6cc7c55d02161fe31a199
• Linux >= 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 < 607e923a3c1b2120de430b3dcde25ed8ad213c0a
• Linux >= 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 < 4ee7fa6cf78ff26d783d39e2949d14c4c1cd5e7f
• Linux 6.11

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.