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

From: Jason Gunthorpe <[email-masked]>
Date: Thu, 28 Oct 2021 08:45:03 -0300

On Wed, Oct 27, 2021 at 06:46:40PM -0700, Jakub Kicinski wrote:
> 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: [email-masked]
> > Signed-off-by: Ziyang Xuan <[email-masked]>
> >  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
> > +++ 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
> > +++ 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.

Isn't it better to reliably return NULL instead of a silent UAF in
this edge case? 

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

RDMA holds a get on the netdev which prevents unregistration, however
unregister_vlan_dev() does:

        unregister_netdevice_queue(dev, head);
        dev_put(real_dev);

Which corrupts the still registered vlan device while it is sitting in
the queue waiting to unregister. So, it is not true that a registered
vlan device always has working vlan_dev_real_dev().

Jason