CVE-2026-23316: net: ipv4: fix ARM64 alignment fault in multipath hash seed

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

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

struct sysctlfibmultipathhashseed contains two u32 fields (userseed and mpseed), making it an 8-byte structure with a 4-byte alignment requirement.

In fibmultipathhashfromkeys(), the code evaluates the entire struct atomically via READONCE():

mpseed = READONCE(net->ipv4.sysctlfibmultipathhashseed).mpseed;

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 READONCE() to acquire when CONFIGLTO=y") strengthens READONCE() 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 READONCE() directly to the u32 member, which emits a safe 32-bit ldar Wn.

Furthermore, Eric Dumazet pointed out that WRITEONCE() on the entire struct in procfibmultipathhashsetseed() 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 WRITEONCE() operations.

Finally, add the missing READONCE() when reading userseed in procfibmultipathhashseed() to ensure proper pairing and concurrency safety.