{"id":"CVE-2026-98134","summary":"bpf: check_cond_jmp_op(): properly infer if register is null","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nbpf: check_cond_jmp_op(): properly infer if register is null\n\nNicholas Carlini reported a bug when verifier can incorrectly infer\nthat a pointer is non-null. The bug occurs when two pointers are\ncompared and one of them has a type w/o PTR_MAYBE_NULL flag,\nbut which allows a value to be NULL at runtime.\nHere is an example:\n\n  // `a` is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED\n  // `a` is 0 at runtime.\n  // `b` is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL\n  void *a = bpf_rdonly_cast(0, 0);\n  int  *b = bpf_map_lookup_elem(...);\n\n  if (a == b)\n    *b = 42;  // verifier does not catch null pointer dereference\n\nThis happens because of a special case in check_cond_jmp_op(),\nwhich attempts to strip PTR_MAYBE_NULL flags from pointer types,\nwhen processing comparisons like `rA == rB`, if either rA or rB can't\nbe null.\n\nThe non-null property is derived based on the absence of\nPTR_MAYBE_NULL flag on rA's or rB's type. But that is not sufficient\nfor types like PTR_TO_MEM, as in the example.\n\nThis patch replaces type_may_be_null() call with reg_not_null(),\nwhich contains an allowlist of types for which absence of\nPTR_MAYBE_NULL actually means that the value can't be NULL at runtime.\n\nAt the moment, the list in the reg_not_null() omits two types for\nwhich PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF.\nIn order to remain backward compatible, and assuming that only\ncomparison between pointers of the same type makes sense,\nthis commit extends reg_not_null(). W/o such an extension e.g.\nverifier_jeq_infer_not_null/null_ptr_to_map_value fails.\n\nreg_not_null() can be extended further, but I deem that out of scope\nfor the fix at hand. Explicit base_type(...) != PTR_TO_BTF_ID\nchecks in the check_cond_jmp_op() can be removed with migration to\nreg_not_null(), but that is a behavioural change, as the special case\nwould start matching for PTR_TO_BTF_ID that is also is_trusted_reg().\nI omit the behavioural change from this commit.","modified":"2026-09-26T03:48:33.323591067Z","published":"2026-09-25T10:36:12.919Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98134.json","cna_assigner":"Linux"},"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/b55c9f019e169ed0d01394f96e172286a8b21b99"},{"type":"WEB","url":"https://git.kernel.org/stable/c/d3ef6c097ba078e1f8c7239d76a0ce8b61e75095"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98134.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98134"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"befae75856ab406a3f3fab2aa2118cf3b2dfe3e6"},{"fixed":"b55c9f019e169ed0d01394f96e172286a8b21b99"},{"fixed":"d3ef6c097ba078e1f8c7239d76a0ce8b61e75095"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98134.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"7.2.7"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98134.json"}}],"schema_version":"1.9.0"}