{
  "affected": [
    {
      "ranges": [
        {
          "events": [
            {
              "introduced": "107e169799057bc6a379ddb625cbe1e51cfc7d72"
            },
            {
              "fixed": "7ce090afbf725e25fff29caacce1eaf8459b1117"
            }
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "type": "GIT"
        }
      ]
    }
  ],
  "database_specific": {
    "cna_assigner": "Linux",
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98869.json"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbpf: Infer zext_dst based on static register liveness analysis\n\nAs reported in the thread [1], the verifier's 32-bit operations zero\nextension logic is broken. This logic is responsible for correct\nsemantics of 32-bit operations on s390 architecture.\n\nAccording to BPF semantics, operation `w1 += 1` is supposed to zero\nextend the upper half of the register `r1`. On s390 the JIT relies on\nthe verifier emitting explicit zero extension before such operations.\n\nThe verifier attempts to minimize the amount of zero extensions\ninserted by tracking whether upper halves of the 64-bit registers are\never used. Previously such tracking worked as follows:\n\n- bpf_reg_state-\u003esubreg_def field was set by do_check_insn()\n  for each operation defining lower but not the upper halves\n  of the register.\n- Whenever an operation reading the whole register was verified,\n  the verifier checked register's subreg_def and set\n  bpf_insn_aux_data-\u003ezext_dst flag as true via a call to\n  mark_insn_zext() function.\n- After the verification was complete, a special pass\n  bpf_opt_subreg_zext_lo32_rnd_hi32() extended 32-bit operations\n  with bpf_insn_aux_data-\u003ezext_dst set as true by adding\n  explicit zero extension.\n\nNote that the logic above relies on bpf_reg_state-\u003esubreg_def,\nwhich is a property of a current verifier state.\nBefore the commit [2] two additional steps happened:\n\n- The verifier tracked upper and lower register halves' liveness as\n  flags REG_LIVE_READ{32,64} in bpf_reg_state-\u003elive.\n- The function propagate_liveness() called mark_insn_zext()\n  in order to transfer the knowledge about which registers have\n  their upper halves alive (and thus might require zero extension).\n\nThe commit [2] removed the two steps described above,\nhence making possible a situation like below:\n\n- The register's upper half is set and is used on some verification\n  path P1 and the register happens not to be marked as precise.\n- The checkpoint C is created while processing some instruction\n  between register initialization and usage.\n- On some other verification path P2 the register's upper half is not\n  initialized and that path ends hitting the checkpoint C.\n- In such a case the register's initialization on path P2 would lack\n  zext_dst mark, making it possible for the program to inject\n  an arbitrary value in the register's upper half.\n\nThis commit replaces subreg_def based logic with computing zext_dst\nstatically, as a part of the bpf_compute_live_registers() analysis:\n\n- The analysis now tracks usage of upper and lower halves of the\n  registers separately.\n- If some instruction defines a 32-bit subregister, but not the whole\n  register, *and* the upper half of the register is alive after that\n  instruction, the instruction is marked as zext_dst.\n\nThere is one notable drop in precision: whenever a BPF subprogram is\ncalled, all 64 bits of parameter registers are presumed to be used.\nThe assumption is that such a drop in precision would not inflict\na noticeable performance penalty.\n\n[1] https://lore.kernel.org/bpf/CAGKGUv=sOuqQtA1Ub-5JXfA4FPosJFYKAQE4B79cK+P1erxqtg@mail.gmail.com/\n[2] commit 107e16979905 (\"bpf: disable and remove registers chain based liveness\")",
  "id": "CVE-2026-98869",
  "modified": "2026-10-22T02:30:29.185958956Z",
  "published": "2026-10-21T14:55:10.364Z",
  "references": [
    {
      "type": "PACKAGE",
      "url": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7ce090afbf725e25fff29caacce1eaf8459b1117"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98869.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98869"
    }
  ],
  "schema_version": "1.9.0",
  "summary": "bpf: Infer zext_dst based on static register liveness analysis"
}