[PATCH net] ipv6: addrconf: validate new MTU before applying it

[PATCH net] ipv6: addrconf: validate new MTU before applying it

[Marcelo Ricardo Leitner]

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>
---
 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

Re: [PATCH net] ipv6: addrconf: validate new MTU before applying it

[Sabrina Dubroca]

Hi,

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

> [...]

> ---
>  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))


> +			*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);
}



Thanks,

-- 
Sabrina

Re: [PATCH net] ipv6: addrconf: validate new MTU before applying it

[Marcelo Ricardo Leitner]

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:
> 
> > [...]
> 
> > ---
> >  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.

> > +			*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