bpf: Mark the zero register precise for a register-form NULL check

{
  "id": "8cebbffcb4e777a2ecb8cd44a14247249afcedff8383d452380f6921b1406b98",
  "url": "https://osv.dev/vulnerability/CVE-2026-98040",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "bpf: Mark the zero register precise for a register-form NULL check",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: bpf: Mark the zero register precise for a register-form NULL check check_cond_jmp_op() accepts \"if rA rB\" as a NULL check for a nullable pointer rA when rB is a scalar known to be zero, lifts PTR_MAYBE_NULL from rA in the corresponding branch and does not mark rB precise. Consider the following program: r0 = bpf_get_prandom_u32(); r6 = 1; /* the r6 == 0 path is explored first */ if (r0 == 0) goto 1f; r6 = 0; 1: r0 = bpf_map_lookup_elem(map, &0); /* absent, NULL at runtime */ if (r0 == r6) goto 2f; /* taken as a NULL check for r0 */ *(u8 *)(r0 + 0); /* verifier: map value; runtime: zero */ 2: return 0; The r6 == 0 path is explored first and the dereference is accepted. The r6 == 1 path is pruned at the checkpoint recorded for (1), so the comparison is never verified with a non-zero r6. At runtime a failed lookup returns NULL, NULL != 1 takes the non-NULL edge and the program dereferences a pointer that is zero.",
    "evidence": "osv:CVE-2026-98040",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "7.2.7",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "CVE-2026-98040",
      "aliases": [
        "CVE-2026-98040"
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "2f4cb53eed448c1aeb6f4b40cf9c810716d8218c"
                },
                {
                  "fixed": "ddca9a3b8833168dda3e62676648a636fb3f221c"
                },
                {
                  "fixed": "6aed0134d3cda6382385a734ae0158eb7df6b142"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98040.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "7.1.0"
                },
                {
                  "fixed": "7.2.7"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98040.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:50.724252296Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/6aed0134d3cda6382385a734ae0158eb7df6b142",
        "https://git.kernel.org/stable/c/ddca9a3b8833168dda3e62676648a636fb3f221c",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98040.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-98040",
        "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/2026/98xxx/CVE-2026-98040.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-98040",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 519448,
  "published_at": "2026-09-25T10:23:53.001+00:00"
}