[PATCH net v3] vlan: enforce underlying device type

[Oscar Maes]

Currently, VLAN devices can be created on top of non-ethernet devices.

Besides the fact that it doesn't make much sense, this also causes a
bug which leaks the address of a kernel function to usermode.

When creating a VLAN device, we initialize GARP (garp_init_applicant)
and MRP (mrp_init_applicant) for the underlying device.

As part of the initialization process, we add the multicast address of
each applicant to the underlying device, by calling dev_mc_add.

__dev_mc_add uses dev->addr_len to determine the length of the new
multicast address.

This causes an out-of-bounds read if dev->addr_len is greater than 6,
since the multicast addresses provided by GARP and MRP are only 6
bytes long.

This behaviour can be reproduced using the following commands:

ip tunnel add gretest mode ip6gre local ::1 remote ::2 dev lo
ip l set up dev gretest
ip link add link gretest name vlantest type vlan id 100

Then, the following command will display the address of garp_pdu_rcv:

ip maddr show | grep 01:80:c2:00:00:21

Fix the bug by enforcing the type of the underlying device during VLAN
device initialization.

Fixes: 22bedad3ce11 ("net: convert multicast list to list_head")
Reported-by: syzbot+91161fe81857b396c8a0@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/netdev/000000000000ca9a81061a01ec20@xxxxxxxxxx/
Signed-off-by: Oscar Maes <oscmaes92@xxxxxxxxx>
---
 net/8021q/vlan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index e45187b88..73a92e748 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -131,7 +131,7 @@ int vlan_check_real_dev(struct net_device *real_dev,
 {
 	const char *name = real_dev->name;
 
-	if (real_dev->features & NETIF_F_VLAN_CHALLENGED) {
+	if (real_dev->features & NETIF_F_VLAN_CHALLENGED || real_dev->type != ARPHRD_ETHER) {
 		pr_info("VLANs not supported on %s\n", name);
 		NL_SET_ERR_MSG_MOD(extack, "VLANs not supported on device");
 		return -EOPNOTSUPP;
-- 
2.39.5