Re: [PATCH net] net: vlan: fix a UAF in vlan_dev_real_dev()

[Jakub Kicinski]

On Wed, 27 Oct 2021 20:16:06 +0800 Ziyang Xuan wrote:
> The real_dev of a vlan net_device may be freed after
> unregister_vlan_dev(). Access the real_dev continually by
> vlan_dev_real_dev() will trigger the UAF problem for the
> real_dev like following:
> 
> ==================================================================
> BUG: KASAN: use-after-free in vlan_dev_real_dev+0xf9/0x120
> Call Trace:
>  kasan_report.cold+0x83/0xdf
>  vlan_dev_real_dev+0xf9/0x120
>  is_eth_port_of_netdev_filter.part.0+0xb1/0x2c0
>  is_eth_port_of_netdev_filter+0x28/0x40
>  ib_enum_roce_netdev+0x1a3/0x300
>  ib_enum_all_roce_netdevs+0xc7/0x140
>  netdevice_event_work_handler+0x9d/0x210
> ...
> 
> Freed by task 9288:
>  kasan_save_stack+0x1b/0x40
>  kasan_set_track+0x1c/0x30
>  kasan_set_free_info+0x20/0x30
>  __kasan_slab_free+0xfc/0x130
>  slab_free_freelist_hook+0xdd/0x240
>  kfree+0xe4/0x690
>  kvfree+0x42/0x50
>  device_release+0x9f/0x240
>  kobject_put+0x1c8/0x530
>  put_device+0x1b/0x30
>  free_netdev+0x370/0x540
>  ppp_destroy_interface+0x313/0x3d0
> ...
> 
> Set vlan->real_dev to NULL after dev_put(real_dev) in
> unregister_vlan_dev(). Check real_dev is not NULL before
> access it in vlan_dev_real_dev().
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+e4df4e1389e28972e955@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
> ---
>  net/8021q/vlan.c      | 1 +
>  net/8021q/vlan_core.c | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 55275ef9a31a..1106da84e725 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -126,6 +126,7 @@ void unregister_vlan_dev(struct net_device *dev, struct list_head *head)
>  
>  	/* Get rid of the vlan's reference to real_dev */
>  	dev_put(real_dev);
> +	vlan->real_dev = NULL;
>  }
>  
>  int vlan_check_real_dev(struct net_device *real_dev,
> diff --git a/net/8021q/vlan_core.c b/net/8021q/vlan_core.c
> index 59bc13b5f14f..343f34479d8b 100644
> --- a/net/8021q/vlan_core.c
> +++ b/net/8021q/vlan_core.c
> @@ -103,7 +103,7 @@ struct net_device *vlan_dev_real_dev(const struct net_device *dev)
>  {
>  	struct net_device *ret = vlan_dev_priv(dev)->real_dev;
>  
> -	while (is_vlan_dev(ret))
> +	while (ret && is_vlan_dev(ret))
>  		ret = vlan_dev_priv(ret)->real_dev;
>  
>  	return ret;

But will make all the callers of vlan_dev_real_dev() feel like they
should NULL-check the result, which is not necessary.

RDMA must be calling this helper on a vlan which was already
unregistered, can we fix RDMA instead?