pppoe: check sockaddr length in pppoe_connect()

BugLink: http://bugs.launchpad.net/bugs/1768474

[ Upstream commit a49e2f5d ]

We must validate sockaddr_len, otherwise userspace can pass fewer data
than we expect and we end up accessing invalid data.

Fixes: 224cf5ad ("ppp: Move the PPP drivers")
Reported-by: syzbot+4f03bdf92fdf9ef5ddab@syzkaller.appspotmail.com
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/ppp/pppoe.c

@@ -638,6 +638,10 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
 	lock_sock(sk);
 
 	error = -EINVAL;
+
+	if (sockaddr_len != sizeof(struct sockaddr_pppox))
+		goto end;
+
 	if (sp->sa_protocol != PX_PROTO_OE)
 		goto end;