[PATCH net] 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 <redacted>
---
 net/ipv6/addrconf.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index f7c8bbeb27b704c0106f714d5a0677c27d3346e0..38892228ccacfe8b67b182784723cc0b67ce572b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c

@@ -4863,6 +4863,36 @@ 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)
+{
+	int *valp = ctl->data;
+	int val = *valp;
+	loff_t pos = *ppos;
+	struct ctl_table lctl;
+	int ret;
+
+	/* ctl->data points to idev->cnf.mtu6
+	 */
+	lctl = *ctl;
+	lctl.data = &val;
+
+	ret = proc_dointvec(&lctl, write, buffer, lenp, ppos);
+
+	if (write) {
+		struct inet6_dev *idev = ctl->extra1;
+
+		if (val >= IPV6_MIN_MTU && val <= idev->dev->mtu)
+			*valp = val;
+		else
+			ret = -EINVAL;
+	}
+	if (ret)
+		*ppos = pos;
+	return ret;
+}
+
 static void dev_disable_change(struct inet6_dev *idev)
 {
 	struct netdev_notifier_info info;

@@ -5014,7 +5044,7 @@ 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",

-- 
1.9.3

Hi,

2015-02-20, 16:24:06 -0200, Marcelo Ricardo Leitner wrote:

[...]

"all" and "default" don't have an idev, so you need a check here:

    if (val >= IPV6_MIN_MTU && (!idev || val <= idev->dev->mtu))

You could call proc_dointvec_minmax to do the checks.  Something like:

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);
}



Thanks,

-- 
Sabrina

On Sat, Feb 21, 2015 at 04:54:07PM +0100, Sabrina Dubroca wrote:

Hi,

2015-02-20, 16:24:06 -0200, Marcelo Ricardo Leitner wrote:

quoted

[...]

quoted

---
 net/ipv6/addrconf.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index f7c8bbeb27b704c0106f714d5a0677c27d3346e0..38892228ccacfe8b67b182784723cc0b67ce572b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c

@@ -4863,6 +4863,36 @@ 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)
+{
+	int *valp = ctl->data;
+	int val = *valp;
+	loff_t pos = *ppos;
+	struct ctl_table lctl;
+	int ret;
+
+	/* ctl->data points to idev->cnf.mtu6
+	 */
+	lctl = *ctl;
+	lctl.data = &val;
+
+	ret = proc_dointvec(&lctl, write, buffer, lenp, ppos);
+
+	if (write) {
+		struct inet6_dev *idev = ctl->extra1;
+
+		if (val >= IPV6_MIN_MTU && val <= idev->dev->mtu)

"all" and "default" don't have an idev, so you need a check here:

    if (val >= IPV6_MIN_MTU && (!idev || val <= idev->dev->mtu))

Oh, indeed.

quoted

+			*valp = val;
+		else
+			ret = -EINVAL;
+	}
+	if (ret)
+		*ppos = pos;
+	return ret;
+}
+

You could call proc_dointvec_minmax to do the checks.  Something like:

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);
}

That's much better! Weird that I did look for a _minmax helper and
missed it :/
Thanks Sabrina, I'll send a v2 soon.

  Marcelo