CVE-2026-98869 (GCVE-0-2026-98869)

Vulnerability from cvelistv5 – Published: 2026-10-21 14:55 – Updated: 2026-10-24 14:09
VLAI
Title
bpf: Infer zext_dst based on static register liveness analysis
Summary
In the Linux kernel, the following vulnerability has been resolved: bpf: Infer zext_dst based on static register liveness analysis As reported in the thread [1], the verifier's 32-bit operations zero extension logic is broken. This logic is responsible for correct semantics of 32-bit operations on s390 architecture. According to BPF semantics, operation `w1 += 1` is supposed to zero extend the upper half of the register `r1`. On s390 the JIT relies on the verifier emitting explicit zero extension before such operations. The verifier attempts to minimize the amount of zero extensions inserted by tracking whether upper halves of the 64-bit registers are ever used. Previously such tracking worked as follows: - bpf_reg_state->subreg_def field was set by do_check_insn() for each operation defining lower but not the upper halves of the register. - Whenever an operation reading the whole register was verified, the verifier checked register's subreg_def and set bpf_insn_aux_data->zext_dst flag as true via a call to mark_insn_zext() function. - After the verification was complete, a special pass bpf_opt_subreg_zext_lo32_rnd_hi32() extended 32-bit operations with bpf_insn_aux_data->zext_dst set as true by adding explicit zero extension. Note that the logic above relies on bpf_reg_state->subreg_def, which is a property of a current verifier state. Before the commit [2] two additional steps happened: - The verifier tracked upper and lower register halves' liveness as flags REG_LIVE_READ{32,64} in bpf_reg_state->live. - The function propagate_liveness() called mark_insn_zext() in order to transfer the knowledge about which registers have their upper halves alive (and thus might require zero extension). The commit [2] removed the two steps described above, hence making possible a situation like below: - The register's upper half is set and is used on some verification path P1 and the register happens not to be marked as precise. - The checkpoint C is created while processing some instruction between register initialization and usage. - On some other verification path P2 the register's upper half is not initialized and that path ends hitting the checkpoint C. - In such a case the register's initialization on path P2 would lack zext_dst mark, making it possible for the program to inject an arbitrary value in the register's upper half. This commit replaces subreg_def based logic with computing zext_dst statically, as a part of the bpf_compute_live_registers() analysis: - The analysis now tracks usage of upper and lower halves of the registers separately. - If some instruction defines a 32-bit subregister, but not the whole register, *and* the upper half of the register is alive after that instruction, the instruction is marked as zext_dst. There is one notable drop in precision: whenever a BPF subprogram is called, all 64 bits of parameter registers are presumed to be used. The assumption is that such a drop in precision would not inflict a noticeable performance penalty. [1] https://lore.kernel.org/bpf/CAGKGUv=sOuqQtA1Ub-5JXfA4FPosJFYKAQE4B79cK+P1erxqtg@mail.gmail.com/ [2] commit 107e16979905 ("bpf: disable and remove registers chain based liveness")
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 7056 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 107e169799057bc6a379ddb625cbe1e51cfc7d72 , < 7ce090afbf725e25fff29caacce1eaf8459b1117 (git)
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Linux Linux Affected: 6.18
Unaffected: 0 , < 6.18 (semver)
Unaffected: 7.3 , ≤ * (original_commit_for_fix)
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "include/linux/bpf_verifier.h",
            "kernel/bpf/fixups.c",
            "kernel/bpf/liveness.c",
            "kernel/bpf/verifier.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "107e169799057bc6a379ddb625cbe1e51cfc7d72",
              "versionType": "git"
            }
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        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "include/linux/bpf_verifier.h",
            "kernel/bpf/fixups.c",
            "kernel/bpf/liveness.c",
            "kernel/bpf/verifier.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "6.18"
            },
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.3",
                  "versionStartIncluding": "6.18",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "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\u0027s 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\u0027s 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\u0027 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\u0027s 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\u0027s upper half is not\n  initialized and that path ends hitting the checkpoint C.\n- In such a case the register\u0027s 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\u0027s 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\")"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - Crafted bytecode enters through bpf(BPF_PROG_LOAD) \u2192 bpf_prog_load() \u2192 bpf_check() \u2192 patched bpf_compute_live_registers(); no network protocol supplies it, and the socket filter is locally loaded and attached.\nAC:L - bpf_is_state_visited() checkpoints attacker-supplied control flow, letting one path fully define R6 and a pruned path execute W6=32 before a 64-bit use; the attacker controls both paths and the retained high bits, with no race or external timing.\nPR:L - bpf_prog_load() permits BPF_PROG_TYPE_SOCKET_FILTER without CAP_BPF when sysctl_unprivileged_bpf_disabled is 0; exploitation was reproduced as uid 1000 with empty capability sets, and BPF tokens separately support namespaced CAP_BPF delegation.\nUI:N - The attacker loads the program, attaches it to an owned socket through SO_ATTACH_BPF, and supplies traffic to execute it; no action by another user is required.\nS:U - The bpf verifier and s390 JIT fault, ARRAY-map out-of-bounds accesses, and resulting current-process credential modification all remain within the host kernel\u0027s security authority; no VM or IOMMU boundary is crossed.\nC:H - The missing zext_dst lets attacker-controlled upper bits form an unchecked 64-bit offset from an ARRAY map-value pointer; the demonstrated exploit disclosed map-\u003eops, derived the KASLR slide, and obtained arbitrary kernel reads.\nI:H - The same unchecked ARRAY map-value offset provides arbitrary kernel writes; the demonstrated exploit located and modified the calling process\u0027s credentials to obtain uid 0.\nA:H - The arbitrary kernel-write primitive created by the omitted s390 zero-extension can corrupt kernel objects or control-flow data, causing an oops or system crash in addition to privilege escalation."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-24T14:09:25.680Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/7ce090afbf725e25fff29caacce1eaf8459b1117"
        }
      ],
      "title": "bpf: Infer zext_dst based on static register liveness analysis",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-98869",
    "datePublished": "2026-10-21T14:55:10.364Z",
    "dateReserved": "2026-09-25T10:25:14.377Z",
    "dateUpdated": "2026-10-24T14:09:25.680Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-98869",
      "date": "2026-10-25",
      "epss": "0.00165",
      "percentile": "0.05708"
    },
    "nvd": {
      "cve": {
        "affected": [
          {
            "affectedData": [
              {
                "defaultStatus": "unaffected",
                "product": "Linux",
                "programFiles": [
                  "include/linux/bpf_verifier.h",
                  "kernel/bpf/fixups.c",
                  "kernel/bpf/liveness.c",
                  "kernel/bpf/verifier.c"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
                    "lessThan": "7ce090afbf725e25fff29caacce1eaf8459b1117",
                    "status": "affected",
                    "version": "107e169799057bc6a379ddb625cbe1e51cfc7d72",
                    "versionType": "git"
                  }
                ]
              },
              {
                "defaultStatus": "affected",
                "product": "Linux",
                "programFiles": [
                  "include/linux/bpf_verifier.h",
                  "kernel/bpf/fixups.c",
                  "kernel/bpf/liveness.c",
                  "kernel/bpf/verifier.c"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
                    "status": "affected",
                    "version": "6.18"
                  },
                  {
                    "lessThan": "6.18",
                    "status": "unaffected",
                    "version": "0",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "*",
                    "status": "unaffected",
                    "version": "7.3",
                    "versionType": "original_commit_for_fix"
                  }
                ]
              }
            ],
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "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\u0027s 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\u0027s 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\u0027 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\u0027s 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\u0027s upper half is not\n  initialized and that path ends hitting the checkpoint C.\n- In such a case the register\u0027s 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\u0027s 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",
        "lastModified": "2026-10-24T14:16:47.630",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.8,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "LOW",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 1.8,
              "impactScore": 5.9,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            }
          ]
        },
        "published": "2026-10-21T15:17:19.827",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/7ce090afbf725e25fff29caacce1eaf8459b1117"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Received"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-10-24T14:22:58+00:00",
      "cve": "CVE-2026-98869",
      "id": "CVE-2026-98869",
      "initial_release_date": "2026-10-21T00:00:00+00:00",
      "product_status:known_affected": "77",
      "product_status:known_not_affected": "201",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: bpf: Infer zext_dst based on static register liveness analysis",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-98869.json",
      "version": "3"
    }
  }
}



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