{"schema":"threatline.record/2","record":{"id":"CVE-2026-98869","aliases":[],"summary":"bpf: Infer zext_dst based on static register liveness analysis","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->subreg_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->zext_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->zext_dst set as true by adding\n  explicit zero extension.\n\nNote that the logic above relies on bpf_reg_state->subreg_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->live.\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\")","categories":["Operating systems"],"product":"Linux","vendor":"Linux","ecosystem":null,"affected_ranges":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98869","source_modified_at":"2026-10-21T14:55:10.364Z","fetched_at":"2026-10-23T18:45:31.882637Z","product":"Linux","vendor":"Linux","versions":[{"version":"107e169799057bc6a379ddb625cbe1e51cfc7d72","lessThan":"7ce090afbf725e25fff29caacce1eaf8459b1117","status":"affected","versionType":"git"}],"defaultStatus":"unaffected"},{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98869","source_modified_at":"2026-10-21T14:55:10.364Z","fetched_at":"2026-10-23T18:45:31.882637Z","product":"Linux","vendor":"Linux","versions":[{"version":"6.18","status":"affected"},{"version":"0","lessThan":"6.18","status":"unaffected","versionType":"semver"},{"version":"7.3","lessThanOrEqual":"*","status":"unaffected","versionType":"original_commit_for_fix"}],"defaultStatus":"affected"}],"fixed_versions":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98869","source_modified_at":"2026-10-21T14:55:10.364Z","fetched_at":"2026-10-23T18:45:31.882637Z","package":{"name":"Linux","ecosystem":null},"version":"7ce090afbf725e25fff29caacce1eaf8459b1117","basis":"first unaffected version stated in the CVE record"}],"cvss":null,"severity":null,"kev":{"listed":false},"epss":{"probability":0.00155,"percentile":0.04312,"score_date":"2026-10-23"},"published_at":"2026-10-21T14:55:10.364Z","modified_at":"2026-10-21T14:55:10.364Z","provenance":[{"source":"CVE List","url":"https://www.cve.org/CVERecord?id=CVE-2026-98869","fetched_at":"2026-10-23T18:45:31.882637Z","source_modified_at":"2026-10-21T14:55:10.364Z"},{"source":"EPSS","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98869","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-23"}],"sources":["CVE List","EPSS"],"references":[{"source":"CVE List","url":"https://git.kernel.org/stable/c/7ce090afbf725e25fff29caacce1eaf8459b1117"}],"locations":[],"motoko_assessment":{"status":"requires_architecture_review","automatic_remediation":false},"cve_state":"PUBLISHED","classification":{"method":"product-first software-layer rules","rules_version":"2.1","matched_on":"product","confidence":"heuristic","reviewed":false},"change_timestamp":"2026-10-21T14:55:10.364Z","content_hash":"98d00a75b613fd63b0cbfddef095ef58ac38051d2435f23c6953d7d1c7f2a2fd"}}