GHSA-7GQJ-379Q-CVHQ

Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-14 15:32
VLAI
Details

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow

xdr_buf_trim() trims len bytes from the tail of an xdr_buf by walking the tail, pages, and head iovecs. Each per-section step uses min_t() so it never removes more bytes than that section holds, but the final accounting at the fix_len label subtracts the total bytes actually consumed from buf->len without any clamp:

fix_len:
        buf->len -= (len - trim);

When the caller has set buf->len to a value smaller than the sum of the iov_lens, (len - trim) can exceed buf->len and the unsigned subtraction wraps to near UINT_MAX. gss_krb5_unwrap_v2() reaches xdr_buf_trim() in exactly that state:

buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);
xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);

buf->len is a small wire-derived value while the iov_lens are at page scale, so the per-section loops legitimately consume far more bytes than buf->len records. The wrapped buf->len then propagates as the authoritative stream bound into every downstream XDR decoder.

Fix by clamping the decrement so buf->len bottoms out at zero:

buf->len -= min_t(unsigned int, buf->len, len - trim);

On the normal path where the iov_lens sum to buf->len, (len - trim) is always <= buf->len and the result is identical to before. No callers change behavior outside the underflow case.

Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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{
  "affected": [],
  "aliases": [
    "CVE-2026-89551"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-09-11T20:19:38Z",
    "severity": "CRITICAL"
  },
  "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.",
  "id": "GHSA-7gqj-379q-cvhq",
  "modified": "2026-09-14T15:32:28Z",
  "published": "2026-09-11T21:31:31Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89551"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b5e37049c"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0c4a8b55e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18d92496ac"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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