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

Overview

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.

Affected

• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < a3d77bcd974b8625d16bddf448cb3a1b5e37049c
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < fa16bbe987b47e771e52eeb1b4540f18d92496ac
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < a924ac4c78afab71bf82641afa3b62e0c4a8b55e
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < e6267cccd7b05cc514e57f2160aa8db85f5c2701
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < ad0cce80d4af2f74674e8b635d97aa3880e83da8
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 85e9602650e9df07190abe817cee3b4d9bc3df17
• Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 3f491306dcb673ff5e78e1044ba450c58978774e
• Linux 3.9

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-16 · vex