# CVE-2026-31429

> Vulnerability.

## Description

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

net: skb: fix cross-cache free of KFENCE-allocated skb head

SKB_SMALL_HEAD_CACHE_SIZE is intentionally set to a non-power-of-2
value (e.g. 704 on x86_64) to avoid collisions with generic kmalloc
bucket sizes. This ensures that skb_kfree_head() can reliably use
skb_end_offset to distinguish skb heads allocated from
skb_small_head_cache vs. generic kmalloc caches.

However, when KFENCE is enabled, kfence_ksize() returns the exact
requested allocation size instead of the slab bucket size. If a caller
(e.g. bpf_test_init) allocates skb head data via kzalloc() and the
requested size happens to equal SKB_SMALL_HEAD_CACHE_SIZE, then
slab_build_skb() -> ksize() returns that exact value. After subtracting
skb_shared_info overhead, skb_end_offset ends up matching
SKB_SMALL_HEAD_HEADROOM, causing skb_kfree_head() to incorrectly free
the object to skb_small_head_cache instead of back to the original
kmalloc cache, resulting in a slab cross-cache free:

  kmem_cache_free(skbuff_small_head): Wrong slab cache. Expected
  skbuff_small_head but got kmalloc-1k

Fix this by always calling kfree(head) in skb_kfree_head(). This keeps
the free path generic and avoids allocator-specific misclassification
for KFENCE objects.

## Key facts

- **CVE ID:** CVE-2026-31429
- **Published:** 2026-04-20

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31429

## Citation

> AI Analytics. CVE-2026-31429. Retrieved 2026-07-20 from https://api.ai-analytics.org/cve/CVE-2026-31429. Derived from NIST NVD. Licensed CC0.

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