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

{
  "id": "5115e8910313f3fcea75aba7593929486e8c58617bbd15580f1d3015ec3e44ea",
  "url": "https://osv.dev/vulnerability/CVE-2026-31429",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "net: skb: fix cross-cache free of KFENCE-allocated skb head",
    "summary": "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.",
    "evidence": "osv:CVE-2026-31429",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.6.136",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.82",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.18.23",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.19.13",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "CVE-2026-31429",
      "aliases": [
        "CVE-2026-31429"
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "bf9f1baa279f0758dc2297080360c5a616843927"
                },
                {
                  "fixed": "128b03ccb2582a643983a48a37fda58df80edbde"
                },
                {
                  "fixed": "60313768a8edc7094435975587c00c2d7b834083"
                },
                {
                  "fixed": "2d64618ea846d8d033477311f805ca487d6a6696"
                },
                {
                  "fixed": "474e00b935db250cac320d10c1d3cf4e44b46721"
                },
                {
                  "fixed": "0f42e3f4fe2a58394e37241d02d9ca6ab7b7d516"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-31429.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.3.0"
                },
                {
                  "fixed": "6.6.136"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.7.0"
                },
                {
                  "fixed": "6.12.82"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.13.0"
                },
                {
                  "fixed": "6.18.23"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.19.0"
                },
                {
                  "fixed": "6.19.13"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-31429.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:49.500794991Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/0f42e3f4fe2a58394e37241d02d9ca6ab7b7d516",
        "https://git.kernel.org/stable/c/128b03ccb2582a643983a48a37fda58df80edbde",
        "https://git.kernel.org/stable/c/2d64618ea846d8d033477311f805ca487d6a6696",
        "https://git.kernel.org/stable/c/474e00b935db250cac320d10c1d3cf4e44b46721",
        "https://git.kernel.org/stable/c/60313768a8edc7094435975587c00c2d7b834083",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31429.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-31429",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31429.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-31429",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 517772,
  "published_at": "2026-04-20T09:43:03.194+00:00"
}