{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-98869","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","state":"PUBLISHED","assignerShortName":"Linux","dateReserved":"2026-09-25T10:25:14.377Z","datePublished":"2026-10-21T14:55:10.364Z","dateUpdated":"2026-10-24T14:09:25.680Z"},"containers":{"cna":{"providerMetadata":{"orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux","dateUpdated":"2026-10-24T14:09:25.680Z"},"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'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\")"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","baseScore":7.8,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:L - Crafted bytecode enters through bpf(BPF_PROG_LOAD) → bpf_prog_load() → bpf_check() → 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's 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->ops, 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's 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."}]}],"affected":[{"product":"Linux","vendor":"Linux","defaultStatus":"unaffected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["include/linux/bpf_verifier.h","kernel/bpf/fixups.c","kernel/bpf/liveness.c","kernel/bpf/verifier.c"],"versions":[{"version":"107e169799057bc6a379ddb625cbe1e51cfc7d72","lessThan":"7ce090afbf725e25fff29caacce1eaf8459b1117","status":"affected","versionType":"git"}]},{"product":"Linux","vendor":"Linux","defaultStatus":"affected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["include/linux/bpf_verifier.h","kernel/bpf/fixups.c","kernel/bpf/liveness.c","kernel/bpf/verifier.c"],"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"}]}],"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.18","versionEndExcluding":"7.3"}]}]}],"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"}}}}