{"id":"CVE-2026-89551","state":"PUBLISHED","title":"SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: xdr_buf_trim: clamp buf->len 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->len without any clamp:\n\n    fix_len:\n            buf->len -= (len - trim);\n\nWhen the caller has set buf->len to a value smaller than the sum\nof the iov_lens, (len - trim) can exceed buf->len and the unsigned\nsubtraction wraps to near UINT_MAX.  gss_krb5_unwrap_v2() reaches\nxdr_buf_trim() in exactly that state:\n\n    buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;\n    buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);\n    xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);\n\nbuf->len 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->len records.  The wrapped buf->len then propagates\nas the authoritative stream bound into every downstream XDR\ndecoder.\n\nFix by clamping the decrement so buf->len bottoms out at zero:\n\n    buf->len -= min_t(unsigned int, buf->len, len - trim);\n\nOn the normal path where the iov_lens sum to buf->len, (len - trim)\nis always <= buf->len and the result is identical to before.  No\ncallers change behavior outside the underflow case.","publishedAt":"2026-09-11T19:44:25.327Z","updatedAt":"2026-10-19T08:48:44.059Z","severity":{"score":9.8,"label":"Critical","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"a3d77bcd974b8625d16bddf448cb3a1b5e37049c","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"fa16bbe987b47e771e52eeb1b4540f18d92496ac","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"a924ac4c78afab71bf82641afa3b62e0c4a8b55e","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"e6267cccd7b05cc514e57f2160aa8db85f5c2701","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"ad0cce80d4af2f74674e8b635d97aa3880e83da8","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"85e9602650e9df07190abe817cee3b4d9bc3df17","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"3f491306dcb673ff5e78e1044ba450c58978774e","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/sunrpc/xdr.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.9","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.9","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/sunrpc/xdr.c"]}],"references":["https://git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b5e37049c","https://git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18d92496ac","https://git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0c4a8b55e","https://git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4","https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701","https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8","https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17","https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:26.108Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/89xxx/CVE-2026-89551.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-89551","state":"PUBLISHED","datePublished":"2026-09-11T19:44:25.327Z","dateUpdated":"2026-10-19T08:48:44.059Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-11T19:38:34.723Z"},"containers":{"cna":{"title":"SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: xdr_buf_trim: clamp buf->len 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->len without any clamp:\n\n    fix_len:\n            buf->len -= (len - trim);\n\nWhen the caller has set buf->len to a value smaller than the sum\nof the iov_lens, (len - trim) can exceed buf->len and the unsigned\nsubtraction wraps to near UINT_MAX.  gss_krb5_unwrap_v2() reaches\nxdr_buf_trim() in exactly that state:\n\n    buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;\n    buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);\n    xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);\n\nbuf->len 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->len records.  The wrapped buf->len then propagates\nas the authoritative stream bound into every downstream XDR\ndecoder.\n\nFix by clamping the decrement so buf->len bottoms out at zero:\n\n    buf->len -= min_t(unsigned int, buf->len, len - trim);\n\nOn the normal path where the iov_lens sum to buf->len, (len - trim)\nis always <= buf->len and the result is identical to before.  No\ncallers change behavior outside the underflow case."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"a3d77bcd974b8625d16bddf448cb3a1b5e37049c","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"fa16bbe987b47e771e52eeb1b4540f18d92496ac","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"a924ac4c78afab71bf82641afa3b62e0c4a8b55e","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"e6267cccd7b05cc514e57f2160aa8db85f5c2701","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"ad0cce80d4af2f74674e8b635d97aa3880e83da8","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"85e9602650e9df07190abe817cee3b4d9bc3df17","versionType":"git"},{"version":"4c190e2f913f038c9c91ee63b59cd037260ba353","status":"affected","lessThan":"3f491306dcb673ff5e78e1044ba450c58978774e","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/sunrpc/xdr.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.9","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.9","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/sunrpc/xdr.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b5e37049c"},{"url":"https://git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18d92496ac"},{"url":"https://git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0c4a8b55e"},{"url":"https://git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4"},{"url":"https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701"},{"url":"https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8"},{"url":"https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17"},{"url":"https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","baseScore":9.8,"baseSeverity":"CRITICAL"},"scenarios":[{"lang":"en","value":"AV:N - xdr_buf_trim() is reached from nfsd RPCSEC_GSS privacy unwrap on TCP/2049 (svcauth_gss_unwrap_priv → gss_unwrap → gss_krb5_unwrap_v2) and from the NFS client's gss_unwrap_resp_priv() on every krb5p reply, so a remote NFS peer delivers the wrap token over the network.\nAC:L - A peer holding the GSS session keys can encrypt a RFC 4121 wrap token so that after decrypt, buf->len is a small wire-derived value while iov_lens remain page-scale; xdr_buf_trim() then subtracts more than buf->len and wraps, with no race or layout outside attacker control.\nPR:N - A malicious or compromised NFS server that already shares a krb5p GSS context needs no account or capability on the victim client; ordinary krb5p replies and NFSv4.0 GSS callbacks are processed without local privileges on that host.\nUI:N - Once a krb5p NFS mount exists, the client unwraps every privacy-protected reply and NFSv4.0 GSS callback with no further mount, click, or open; idle revalidations and server-driven callbacks are sufficient.\nS:U - The wrapped buf->len and subsequent XDR decode run in the host kernel that processed the RPC and do not cross a VM, IOMMU, or sandbox boundary.\nC:H - The underflowed buf->len (~UINT_MAX) becomes the authoritative stream bound, and on nfsd xdr->nwords also underflows via saved_len - buf->len, so later xdr_read_pages/xdr_shrink_bufhead and NFS XDR decoders read far past the receive buffer into adjacent kernel memory.\nI:H - xdr_shrink_bufhead() calls xdr_buf_head_shift_right() with a near-UINT_MAX length derived from the wrapped buf->len, and garbage xdr_stream_pos values can index the svc pages[] array out of bounds, an out-of-bounds write exploitable for control-flow hijacking.\nA:H - Those unbounded XDR copies and the inconsistent xdr_buf (wrapped length versus iov_lens) oops or panic nfsd or the NFS client, and the peer can repeat the request at will."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-19T08:48:44.059Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"5.10.270"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"5.15.221"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"6.1.188"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"6.6.157"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"6.12.109"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"6.18.50"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"7.2.4"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.9","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-89551","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"total"}],"version":"2.0.3","timestamp":"2026-10-16T19:23:51.789342Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-16T19:26:43.285Z"}}]}}}