[PATCH net v3] vlan: enforce underlying device type

[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@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/000000000000ca9a81061a01ec20@google.com/
Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
---
 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

Re: [PATCH net v3] vlan: enforce underlying device type

[Jiri Pirko]

Mon, Mar 03, 2025 at 04:56:19PM +0100, oscmaes92@gmail.com wrote:
>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@syzkaller.appspotmail.com
>Closes: https://lore.kernel.org/netdev/000000000000ca9a81061a01ec20@google.com/
>Signed-off-by: Oscar Maes <oscmaes92@gmail.com>


Reviewed-by: Jiri Pirko <jiri@nvidia.com>

Re: [PATCH net v3] vlan: enforce underlying device type

[Jakub Kicinski]

On Mon,  3 Mar 2025 16:56:19 +0100 Oscar Maes wrote:
> 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.

Applied, with the line wrapped at 80 col, thanks!