[PATCH net] macsec: avoid to set wrong mtu

[PATCH net] macsec: avoid to set wrong mtu

[Taehee Yoo]

When a macsec interface is created, the mtu is calculated with the lower
interface's mtu value.
If the mtu of lower interface is lower than the length, which is needed
by macsec interface, macsec's mtu value will be overflowed.
So, if the lower interface's mtu is too low, macsec interface's mtu
should be set to 0.

Test commands:
    ip link add dummy0 mtu 10 type dummy
    ip link add macsec0 link dummy0 type macsec
    ip link show macsec0

Before:
    11: macsec0@dummy0: <BROADCAST,MULTICAST,M-DOWN> mtu 4294967274
After:
    11: macsec0@dummy0: <BROADCAST,MULTICAST,M-DOWN> mtu 0

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
---
 drivers/net/macsec.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index a183250ff66a..758baf7cb8a1 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -4002,11 +4002,11 @@ static int macsec_newlink(struct net *net, struct net_device *dev,
 			  struct netlink_ext_ack *extack)
 {
 	struct macsec_dev *macsec = macsec_priv(dev);
+	rx_handler_func_t *rx_handler;
+	u8 icv_len = DEFAULT_ICV_LEN;
 	struct net_device *real_dev;
-	int err;
+	int err, mtu;
 	sci_t sci;
-	u8 icv_len = DEFAULT_ICV_LEN;
-	rx_handler_func_t *rx_handler;
 
 	if (!tb[IFLA_LINK])
 		return -EINVAL;
@@ -4033,7 +4033,11 @@ static int macsec_newlink(struct net *net, struct net_device *dev,
 
 	if (data && data[IFLA_MACSEC_ICV_LEN])
 		icv_len = nla_get_u8(data[IFLA_MACSEC_ICV_LEN]);
-	dev->mtu = real_dev->mtu - icv_len - macsec_extra_len(true);
+	mtu = real_dev->mtu - icv_len - macsec_extra_len(true);
+	if (mtu < 0)
+		dev->mtu = 0;
+	else
+		dev->mtu = mtu;
 
 	rx_handler = rtnl_dereference(real_dev->rx_handler);
 	if (rx_handler && rx_handler != macsec_handle_frame)
-- 
2.17.1

Re: [PATCH net] macsec: avoid to set wrong mtu

[David Miller]

From: Taehee Yoo <ap420073@gmail.com>
Date: Thu, 23 Apr 2020 13:40:47 +0000

> When a macsec interface is created, the mtu is calculated with the lower
> interface's mtu value.
> If the mtu of lower interface is lower than the length, which is needed
> by macsec interface, macsec's mtu value will be overflowed.
> So, if the lower interface's mtu is too low, macsec interface's mtu
> should be set to 0.
> 
> Test commands:
>     ip link add dummy0 mtu 10 type dummy
>     ip link add macsec0 link dummy0 type macsec
>     ip link show macsec0
> 
> Before:
>     11: macsec0@dummy0: <BROADCAST,MULTICAST,M-DOWN> mtu 4294967274
> After:
>     11: macsec0@dummy0: <BROADCAST,MULTICAST,M-DOWN> mtu 0
> 
> Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
> Signed-off-by: Taehee Yoo <ap420073@gmail.com>

Applied and queued up for -stable.