CVE-2026-23316
Status: AnalyzedMedium (5.5)—
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()`:
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.
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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.
Technical details traces, logs and code from the original report
mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;
CVSS
- Version: 3.1
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- Base score: 5.5
Exploitation probability (EPSS)
- Probability of exploitation in the next 30 days: 0.12%
- Percentile among all scored CVEs: 2
- Score date: 10/5/2026
EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).
Affected technologies (1)
CWEs
- NVD-CWE-noinfo
References
Raw JSON (NVD)
Show
{
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"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: ipv4: fix ARM64 alignment fault in multipath hash seed\n\n`struct sysctl_fib_multipath_hash_seed` contains two u32 fields\n(user_seed and mp_seed), making it an 8-byte structure with a 4-byte\nalignment requirement.\n\nIn `fib_multipath_hash_from_keys()`, the code evaluates the entire\nstruct atomically via `READ_ONCE()`:\n\n mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;\n\nWhile this silently works on GCC by falling back to unaligned regular\nloads which the ARM64 kernel tolerates, it causes a fatal kernel panic\nwhen compiled with Clang and LTO enabled.\n\nCommit e35123d83ee3 (\"arm64: lto: Strengthen READ_ONCE() to acquire\nwhen CONFIG_LTO=y\") strengthens `READ_ONCE()` to use Load-Acquire\ninstructions (`ldar` / `ldapr`) to prevent compiler reordering bugs\nunder Clang LTO. Since the macro evaluates the full 8-byte struct,\nClang emits a 64-bit `ldar` instruction. ARM64 architecture strictly\nrequires `ldar` to be naturally aligned, thus executing it on a 4-byte\naligned address triggers a strict Alignment Fault (FSC = 0x21).\n\nFix the read side by moving the `READ_ONCE()` directly to the `u32`\nmember, which emits a safe 32-bit `ldar Wn`.\n\nFurthermore, Eric Dumazet pointed out that `WRITE_ONCE()` on the entire\nstruct in `proc_fib_multipath_hash_set_seed()` is also flawed. Analysis\nshows that Clang splits this 8-byte write into two separate 32-bit\n`str` instructions. While this avoids an alignment fault, it destroys\natomicity and exposes a tear-write vulnerability. Fix this by\nexplicitly splitting the write into two 32-bit `WRITE_ONCE()`\noperations.\n\nFinally, add the missing `READ_ONCE()` when reading `user_seed` in\n`proc_fib_multipath_hash_seed()` to ensure proper pairing and\nconcurrency safety."
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"value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nnet: ipv4: solucionar fallo de alineación ARM64 en la semilla de hash multipath\n\n'struct sysctl_fib_multipath_hash_seed' contiene dos campos u32 (user_seed y mp_seed), convirtiéndola en una estructura de 8 bytes con un requisito de alineación de 4 bytes.\n\nEn 'fib_multipath_hash_from_keys()', el código evalúa la estructura completa atómicamente a través de 'READ_ONCE()':\n\n mp_seed = 'READ_ONCE'(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;\n\nAunque esto funciona silenciosamente en GCC al recurrir a cargas regulares no alineadas que el kernel ARM64 tolera, causa un pánico fatal del kernel cuando se compila con Clang y LTO habilitado.\n\nEl commit e35123d83ee3 ('arm64: lto: Strengthen 'READ_ONCE()' to acquire when CONFIG_LTO=y') refuerza 'READ_ONCE()' para usar instrucciones Load-Acquire ('ldar' / 'ldapr') para prevenir errores de reordenamiento del compilador bajo Clang LTO. Dado que la macro evalúa la estructura completa de 8 bytes, Clang emite una instrucción 'ldar' de 64 bits. La arquitectura ARM64 requiere estrictamente que 'ldar' esté naturalmente alineado, por lo tanto, ejecutarlo en una dirección alineada a 4 bytes desencadena un fallo de alineación estricto (FSC = 0x21).\n\nSolucionar el lado de lectura moviendo 'READ_ONCE()' directamente al miembro 'u32', lo que emite un 'ldar Wn' seguro de 32 bits.\n\nAdemás, Eric Dumazet señaló que 'WRITE_ONCE()' en la estructura completa en 'proc_fib_multipath_hash_set_seed()' también es defectuoso. El análisis muestra que Clang divide esta escritura de 8 bytes en dos instrucciones 'str' separadas de 32 bits. Aunque esto evita un fallo de alineación, destruye la atomicidad y expone una vulnerabilidad de escritura fragmentada. Solucionar esto dividiendo explícitamente la escritura en dos operaciones 'WRITE_ONCE()' de 32 bits.\n\nFinalmente, añadir el 'READ_ONCE()' faltante al leer 'user_seed' en 'proc_fib_multipath_hash_seed()' para asegurar un emparejamiento adecuado y seguridad de concurrencia."
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"lastModified": "2026-06-17T10:21:18.943",
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