{"id":"CVE-2026-89551","summary":"SUNRPC: xdr_buf_trim: clamp buf-\u003elen to avoid underflow","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: xdr_buf_trim: clamp buf-\u003elen to avoid underflow\n\nxdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by\nwalking the tail, pages, and head iovecs.  Each per-section step\nuses min_t() so it never removes more bytes than that section\nholds, but the final accounting at the fix_len label subtracts the\ntotal bytes actually consumed from buf-\u003elen without any clamp:\n\n    fix_len:\n            buf-\u003elen -= (len - trim);\n\nWhen the caller has set buf-\u003elen to a value smaller than the sum\nof the iov_lens, (len - trim) can exceed buf-\u003elen and the unsigned\nsubtraction wraps to near UINT_MAX.  gss_krb5_unwrap_v2() reaches\nxdr_buf_trim() in exactly that state:\n\n    buf-\u003ehead[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;\n    buf-\u003elen = len - (GSS_KRB5_TOK_HDR_LEN + headskip);\n    xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);\n\nbuf-\u003elen is a small wire-derived value while the iov_lens are at\npage scale, so the per-section loops legitimately consume far more\nbytes than buf-\u003elen records.  The wrapped buf-\u003elen then propagates\nas the authoritative stream bound into every downstream XDR\ndecoder.\n\nFix by clamping the decrement so buf-\u003elen bottoms out at zero:\n\n    buf-\u003elen -= min_t(unsigned int, buf-\u003elen, len - trim);\n\nOn the normal path where the iov_lens sum to buf-\u003elen, (len - trim)\nis always \u003c= buf-\u003elen and the result is identical to before.  No\ncallers change behavior outside the underflow case.","modified":"2026-09-13T03:47:19.217017308Z","published":"2026-09-11T19:44:25.327Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89551.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e"},{"type":"WEB","url":"https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17"},{"type":"WEB","url":"https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8"},{"type":"WEB","url":"https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89551.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-89551"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"4c190e2f913f038c9c91ee63b59cd037260ba353"},{"fixed":"e6267cccd7b05cc514e57f2160aa8db85f5c2701"},{"fixed":"ad0cce80d4af2f74674e8b635d97aa3880e83da8"},{"fixed":"85e9602650e9df07190abe817cee3b4d9bc3df17"},{"fixed":"3f491306dcb673ff5e78e1044ba450c58978774e"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89551.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.9.0"},{"fixed":"6.12.109"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.50"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.4"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89551.json"}}],"schema_version":"1.9.0"}