ipv6: addrconf: validate new MTU before applying it

commit 77751427 upstream.

Currently we don't check if the new MTU is valid or not and this allows
one to configure a smaller than minimum allowed by RFCs or even bigger
than interface own MTU, which is a problem as it may lead to packet
drops.

If you have a daemon like NetworkManager running, this may be exploited
by remote attackers by forging RA packets with an invalid MTU, possibly
leading to a DoS. (NetworkManager currently only validates for values
too small, but not for too big ones.)

The fix is just to make sure the new value is valid. That is, between
IPV6_MIN_MTU and interface's MTU.

Note that similar check is already performed at
ndisc_router_discovery(), for when kernel itself parses the RA.
Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 2.6.32:
 - Add a strategy for the sysctl as we don't get a default strategy
 - Adjust context, spacing]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>

net/ipv6/addrconf.c

@@ -4036,6 +4036,40 @@ static int addrconf_sysctl_forward_strategy(ctl_table *table,
 	return addrconf_fixup_forwarding(table, valp, val);
 }
 
+static
+struct ctl_table *addrconf_sysctl_mtu_init(struct ctl_table *newctl,
+					   const struct ctl_table *ctl)
+{
+	struct inet6_dev *idev = ctl->extra1;
+	static int min_mtu = IPV6_MIN_MTU;
+
+	*newctl = *ctl;
+	newctl->extra1 = &min_mtu;
+	newctl->extra2 = idev ? &idev->dev->mtu : NULL;
+	return newctl;
+}
+
+static
+int addrconf_sysctl_mtu(struct ctl_table *ctl, int write,
+			void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct ctl_table lctl;
+
+	return proc_dointvec_minmax(addrconf_sysctl_mtu_init(&lctl, ctl),
+				    write, buffer, lenp, ppos);
+}
+
+static int addrconf_sysctl_mtu_strategy(struct ctl_table *ctl,
+					void __user *oldval,
+					size_t __user *oldlenp,
+					void __user *newval, size_t newlen)
+{
+	struct ctl_table lctl;
+
+	return sysctl_intvec(addrconf_sysctl_mtu_init(&lctl, ctl),
+			     oldval, oldlenp, newval, newlen);
+}
+
 static void dev_disable_change(struct inet6_dev *idev)
 {
 	if (!idev || !idev->dev)
@@ -4142,7 +4176,8 @@ static struct addrconf_sysctl_table
 			.data		=	&ipv6_devconf.mtu6,
 			.maxlen		=	sizeof(int),
 			.mode		=	0644,
-			.proc_handler	=	proc_dointvec,
+			.proc_handler	=	addrconf_sysctl_mtu,
+			.strategy	=	addrconf_sysctl_mtu_strategy,
 		},
 		{
 			.ctl_name	=	NET_IPV6_ACCEPT_RA,