SUNRPC: Add an XDR decoding helper for struct opaque_auth

RFC 5531 defines the body of an RPC Call message like this:

	struct call_body {
		unsigned int rpcvers;
		unsigned int prog;
		unsigned int vers;
		unsigned int proc;
		opaque_auth cred;
		opaque_auth verf;
		/* procedure-specific parameters start here */
	};

In the current server code, decoding a struct opaque_auth type is
open-coded in several places, and is thus difficult to harden
everywhere.

Introduce a helper for decoding an opaque_auth within the context
of a xdr_stream. This helper can be shared with all authentication
flavor implemenations, even on the client-side.

Done as part of hardening the server-side RPC header decoding paths.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/xdr.h

@@ -346,6 +346,9 @@ ssize_t xdr_stream_decode_string(struct xdr_stream *xdr, char *str,
 		size_t size);
 ssize_t xdr_stream_decode_string_dup(struct xdr_stream *xdr, char **str,
 		size_t maxlen, gfp_t gfp_flags);
+ssize_t xdr_stream_decode_opaque_auth(struct xdr_stream *xdr, u32 *flavor,
+		void **body, unsigned int *body_len);
+
 /**
  * xdr_align_size - Calculate padded size of an object
  * @n: Size of an object being XDR encoded (in bytes)

net/sunrpc/xdr.c

@@ -2274,3 +2274,31 @@ ssize_t xdr_stream_decode_string_dup(struct xdr_stream *xdr, char **str,
 	return ret;
 }
 EXPORT_SYMBOL_GPL(xdr_stream_decode_string_dup);
+
+/**
+ * xdr_stream_decode_opaque_auth - Decode struct opaque_auth (RFC5531 S8.2)
+ * @xdr: pointer to xdr_stream
+ * @flavor: location to store decoded flavor
+ * @body: location to store decode body
+ * @body_len: location to store length of decoded body
+ *
+ * Return values:
+ *   On success, returns the number of buffer bytes consumed
+ *   %-EBADMSG on XDR buffer overflow
+ *   %-EMSGSIZE if the decoded size of the body field exceeds 400 octets
+ */
+ssize_t xdr_stream_decode_opaque_auth(struct xdr_stream *xdr, u32 *flavor,
+				      void **body, unsigned int *body_len)
+{
+	ssize_t ret, len;
+
+	len = xdr_stream_decode_u32(xdr, flavor);
+	if (unlikely(len < 0))
+		return len;
+	ret = xdr_stream_decode_opaque_inline(xdr, body, RPC_MAX_AUTH_SIZE);
+	if (unlikely(ret < 0))
+		return ret;
+	*body_len = ret;
+	return len + ret;
+}
+EXPORT_SYMBOL_GPL(xdr_stream_decode_opaque_auth);