can: raw: return -ERANGE when filterset does not fit into user space buffer

Multiple filters (struct can_filter) can be set with the setsockopt()
function, which was originally intended as a write-only operation.

As getsockopt() also provides a CAN_RAW_FILTER option to read back the
given filters, the caller has to provide an appropriate user space buffer.
In the case this buffer is too small the getsockopt() silently truncates
the filter information and gives no information about the needed space.
This is safe but not convenient for the programmer.

In net/core/sock.c the SO_PEERGROUPS sockopt had a similar requirement
and solved it by returning -ERANGE in the case that the provided data
does not fit into the given user space buffer and fills the required size
into optlen, so that the caller can retry with a matching buffer length.

This patch adopts this approach for CAN_RAW_FILTER getsockopt().
Reported-by: Phillip Schichtel <phillip@schich.tel>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Tested-By: Phillip Schichtel <phillip@schich.tel>
Link: https://lore.kernel.org/r/20201216174928.21663-1-socketcan@hartkopp.netSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/raw.c

@@ -665,10 +665,18 @@ static int raw_getsockopt(struct socket *sock, int level, int optname,
 		if (ro->count > 0) {
 			int fsize = ro->count * sizeof(struct can_filter);
 
-			if (len > fsize)
-				len = fsize;
-			if (copy_to_user(optval, ro->filter, len))
-				err = -EFAULT;
+			/* user space buffer to small for filter list? */
+			if (len < fsize) {
+				/* return -ERANGE and needed space in optlen */
+				err = -ERANGE;
+				if (put_user(fsize, optlen))
+					err = -EFAULT;
+			} else {
+				if (len > fsize)
+					len = fsize;
+				if (copy_to_user(optval, ro->filter, len))
+					err = -EFAULT;
+			}
 		} else {
 			len = 0;
 		}