Vulnerability: net/ipv6/addrconf.c in the IPv6 stack in the Linux kernel before 4.0 does not validate attempted changes to the MTU value, which allows context-dependent attackers to cause a denial of service (packet loss) via a value that is (1) smaller than the minimum compliant value or (2) larger than the MTU of an interface, as demonstrated by a Router Advertisement (RA) message that is not validated by a daemon, a different vulnerability than CVE-2015-0272.  NOTE: the scope of CVE-2015-0272 is limited to the NetworkManager product.
Vulnerability ID: CVE-2015-8215
CWE: CWE-20
Severity score: 2.9
Project: https://github.com/torvalds/linux
Published: 2015-11-16
File: net/ipv6/addrconf.c
Commit message: ipv6: addrconf: validate new MTU before applying it

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>
Commit author: Marcelo Leitner

Vulnerable version (before fix):
 int addrconf_sysctl_forward(struct ctl_table *ctl, int write, 	return ret; }  static void dev_disable_change(struct inet6_dev *idev) { 	struct netdev_notifier_info info; static struct addrconf_sysctl_table 			.data		= &ipv6_devconf.mtu6, 			.maxlen		= sizeof(int), 			.mode		= 0644, 			.proc_handler	= proc_dointvec, 		}, 		{ 			.procname	= "accept_ra",

Fixed version (after fix):
 int addrconf_sysctl_forward(struct ctl_table *ctl, int write, 	return ret; }  static int addrconf_sysctl_mtu(struct ctl_table *ctl, int write, 			void __user *buffer, size_t *lenp, loff_t *ppos) { 	struct inet6_dev *idev = ctl->extra1; 	int min_mtu = IPV6_MIN_MTU; 	struct ctl_table lctl;  	lctl = *ctl; 	lctl.extra1 = &min_mtu; 	lctl.extra2 = idev ? &idev->dev->mtu : NULL;  	return proc_dointvec_minmax(&lctl, write, buffer, lenp, ppos); }  static void dev_disable_change(struct inet6_dev *idev) { 	struct netdev_notifier_info info; static struct addrconf_sysctl_table 			.data		= &ipv6_devconf.mtu6, 			.maxlen		= sizeof(int), 			.mode		= 0644, 			.proc_handler	= addrconf_sysctl_mtu, 		}, 		{ 			.procname	= "accept_ra",

Patch:
patch: @@ -4903,6 +4903,21 @@ int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
 	return ret;
 }
 
+static
+int addrconf_sysctl_mtu(struct ctl_table *ctl, int write,
+			void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct inet6_dev *idev = ctl->extra1;
+	int min_mtu = IPV6_MIN_MTU;
+	struct ctl_table lctl;
+
+	lctl = *ctl;
+	lctl.extra1 = &min_mtu;
+	lctl.extra2 = idev ? &idev->dev->mtu : NULL;
+
+	return proc_dointvec_minmax(&lctl, write, buffer, lenp, ppos);
+}
+
 static void dev_disable_change(struct inet6_dev *idev)
 {
 	struct netdev_notifier_info info;
@@ -5054,7 +5069,7 @@ static struct addrconf_sysctl_table
 			.data		= &ipv6_devconf.mtu6,
 			.maxlen		= sizeof(int),
 			.mode		= 0644,
-			.proc_handler	= proc_dointvec,
+			.proc_handler	= addrconf_sysctl_mtu,
 		},
 		{
 			.procname	= "accept_ra",
path: net/ipv6/addrconf.c