CVE-2015-8215: Improper Input Validation (CWE-20)
value: 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.
CWE description: The product receives input or data, but it does
        not validate or incorrectly validates that the input has the
        properties that are required to process the data safely and
        correctly.
Published: 2015-11-16T21:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/ipv6/addrconf.c
Commit date: 2015-02-23 18:16:12 -0500
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>

Diff: diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 98e4a63d72bb43..b6030025f41197 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -4901,10 +4901,25 @@ int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
 	if (ret)
 		*ppos = pos;
 	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;
 
 	if (!idev || !idev->dev)
@@ -5052,11 +5067,11 @@ static struct addrconf_sysctl_table
 		{
 			.procname	= "mtu",
 			.data		= &ipv6_devconf.mtu6,
 			.maxlen		= sizeof(int),
 			.mode		= 0644,
-			.proc_handler	= proc_dointvec,
+			.proc_handler	= addrconf_sysctl_mtu,
 		},
 		{
 			.procname	= "accept_ra",
 			.data		= &ipv6_devconf.accept_ra,
 			.maxlen		= sizeof(int),

Vulnerable code: 			.proc_handler	= proc_dointvec,
Fixed code: 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);
}

			.proc_handler	= addrconf_sysctl_mtu,