Subject: Re: [PATCH 1/3] dev: Prevent creating network devices with negative ifindex

From: Stephen Hemminger <[email-masked]>
Date: Fri, 16 Jun 2017 10:25:42 -0700

On Fri, 16 Jun 2017 19:44:45 +0300
Serhey Popovych <[email-masked]> wrote:

> > On Fri, 16 Jun 2017 17:23:51 +0300
> > Serhey Popovych <[email-masked]> wrote:
> >   
> >> Interface index is signed integer, we can pass ifm->ifi_index
> >> from userspace via netlink and create network device with
> >> negative ifindex value.
> >>
> >> Fixes: 9c7dafbfab15 ("net: Allow to create links with given ifindex")
> >> Signed-off-by: Serhey Popovych <[email-masked]>
> >> ---
> >>  net/core/dev.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> index 8658074..dae8010 100644
> >> --- a/net/core/dev.c
> >> +++ b/net/core/dev.c
> >> @@ -7491,7 +7491,7 @@ int register_netdevice(struct net_device *dev)
> >>  	}
> >>  
> >>  	ret = -EBUSY;
> >> -	if (!dev->ifindex)
> >> +	if (dev->ifindex <= 0)
> >>  		dev->ifindex = dev_new_index(net);
> >>  	else if (__dev_get_by_index(net, dev->ifindex))
> >>  		goto err_uninit;  
> > 
> > You should fix this by adding error check in the netlink portion
> > that allows creating devices with given ifindex. Passing < 0
> > should be an error.But should this break some setups if I add such check to netlink  
> portion? In my opinion it is better to choose silently different
> ifindex rather than reporting failure. That's why I prefer doing
> this in register_netdevice().
> 
> Also there is similar problem for drivers/net/veth.c, it might
> happen that other places will be added later that setup
> dev->ifindex and then call register_netdevice().
> 
> What do you think?

Passing -1 is an error, it doesn't make sense  to try and be
helpful to buggy userland.