NFSD: Prevent a potential integer overflow

If the tag length is >= U32_MAX - 3 then the "length + 4" addition
can result in an integer overflow. Address this by splitting the
decoding into several steps so that decode_cb_compound4res() does
not have to perform arithmetic on the unsafe length value.

Reported-by: Dan Carpenter <[e-mail]>
Cc: [e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4callback.c

···
 	u32 length;
 	__be32 *p;
 
-	p = xdr_inline_decode(xdr, 4 + 4);
+	p = xdr_inline_decode(xdr, XDR_UNIT);
 	if (unlikely(p == NULL))
 		goto out_overflow;
-	hdr->status = be32_to_cpup(p++);
+	hdr->status = be32_to_cpup(p);
 	/* Ignore the tag */
-	length = be32_to_cpup(p++);
-	p = xdr_inline_decode(xdr, length + 4);
-	if (unlikely(p == NULL))
+	if (xdr_stream_decode_u32(xdr, &length) < 0)
 		goto out_overflow;
-	p += XDR_QUADLEN(length);
-	hdr->nops = be32_to_cpup(p);
+	if (xdr_inline_decode(xdr, length) == NULL)
+		goto out_overflow;
+	if (xdr_stream_decode_u32(xdr, &hdr->nops) < 0)
+		goto out_overflow;
 	return 0;
 out_overflow:
 	return -EIO;