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.

Fixes: 4c190e2f91 ("sunrpc: trim off trailing checksum before returning decrypted or integrity authenticated buffer")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260524010213.557424-4-cel@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xdr.c

@@ -2049,7 +2049,7 @@ void xdr_buf_trim(struct xdr_buf *buf, unsigned int len)
 		trim -= cur;
 	}
 fix_len:
-	buf->len -= (len - trim);
+	buf->len -= min_t(unsigned int, buf->len, len - trim);
 }
 EXPORT_SYMBOL_GPL(xdr_buf_trim);