CVE-2026-89551

Summary

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 Software

Weaknesses

References

• 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

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