net: netprio: fd passed in SCM_RIGHTS datagram not set correctly

A socket fd passed in a SCM_RIGHTS datagram was not getting
updated with the new tasks cgrp prioidx. This leaves IO on
the socket tagged with the old tasks priority.

To fix this add a check in the scm recvmsg path to update the
sock cgrp prioidx with the new tasks value.

Thanks to Al Viro for catching this.

CC: Neil Horman <nhorman@tuxdriver.com>
Reported-by: NAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NJohn Fastabend <john.r.fastabend@intel.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/scm.c

@@ -265,6 +265,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 	for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
 	     i++, cmfptr++)
 	{
+		struct socket *sock;
 		int new_fd;
 		err = security_file_receive(fp[i]);
 		if (err)
@@ -281,6 +282,9 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 		}
 		/* Bump the usage count and install the file. */
 		get_file(fp[i]);
+		sock = sock_from_file(fp[i], &err);
+		if (sock)
+			sock_update_netprioidx(sock->sk, current);
 		fd_install(new_fd, fp[i]);
 	}