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Subject[PATCH 3.16.y-ckt 001/133] ipv6: addrconf: validate new MTU before applying it
3.16.7-ckt18 -stable review patch.  If anyone has any objections, please let me know.


From: Marcelo Leitner <>

commit 77751427a1ff25b27d47a4c36b12c3c8667855ac upstream.

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

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 <>
Signed-off-by: Sabrina Dubroca <>
Signed-off-by: David S. Miller <>
Cc: Moritz Muehlenhoff <>
Signed-off-by: Luis Henriques <>
net/ipv6/addrconf.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 4a9a34954923..75630e173adf 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -4789,6 +4789,21 @@ int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
return ret;

+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;
@@ -4940,7 +4955,7 @@ 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",

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