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>

1 files changed, 16 insertions, 1 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 98e4a63d72bb..b6030025f411 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -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",