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

Description

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.

Timeline

  1. ID reserved

    By Linux

  2. · 2 months later

    Published

  3. · 8 months later

    Record updated

Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.

Scores

CVSS scores of CVE-2026-23316, by who scored it
Scored byCVSSScoreVector
NVDNVD · Analyzed3.1Medium5.5CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS (exploitation in 30 days)
0.13% · top 97% of all CVEs (15 Nov 2026)
CISA SSVC
Exploitation: none · Automatable: no · Technical impact: partial
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 5.5.

Affected products

  • Linux · Linux

    • affected 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 – < 4bdc94d45d5459f0149085dfc1efe733c8e14f11
    • affected 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 – < 7e4ad34a8889a6a9e0f6cc7c55d02161fe31a199
    • affected 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 – < 607e923a3c1b2120de430b3dcde25ed8ad213c0a
    • affected 4ee2a8cace3fb9a34aea6a56426f89d26dd514f3 – < 4ee7fa6cf78ff26d783d39e2949d14c4c1cd5e7f

  • Linux · Linux

    • affected 6.11
    • unaffected < 6.11
    • unaffected 6.12.77 – ≤ 6.12.*
    • unaffected 6.18.17 – ≤ 6.18.*
    • unaffected 6.19.7 – ≤ 6.19.*
    • unaffected 7.0 – ≤ *

References

References4

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .