CVE-2026-89551

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

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

xdrbuftrim() 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 UINTMAX.  gsskrb5unwrapv2() reaches

xdrbuftrim() in exactly that state:

    buf->head[0].iovlen -= GSSKRB5TOKHDR_LEN + headskip;

    buf->len = len - (GSSKRB5TOKHDRLEN + headskip);

    xdrbuftrim(buf, ec + GSSKRB5TOKHDRLEN + 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.