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

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 <serhe.popovych@gmail.com>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Fri, 16 Jun 2017 17:23:51 +0300
Serhey Popovych <serhe.popovych@gmail.com> 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 <serhe.popovych@gmail.com>
> ---
>  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.

> On Fri, 16 Jun 2017 17:23:51 +0300
> Serhey Popovych <serhe.popovych@gmail.com> 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 <serhe.popovych@gmail.com>
>> ---
>>  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?

Thanks for quick review!

>

On Fri, 16 Jun 2017 19:44:45 +0300
Serhey Popovych <serhe.popovych@gmail.com> wrote:

> > On Fri, 16 Jun 2017 17:23:51 +0300
> > Serhey Popovych <serhe.popovych@gmail.com> 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 <serhe.popovych@gmail.com>
> >> ---
> >>  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.

>> What do you think?
>
> Passing -1 is an error, it doesn't make sense  to try and be
> helpful to buggy userland.

Here is commit I actually change/fix:

commit 9c7dafbfab15 ("net: Allow to create links with given ifindex")

In this change done the opposite: check for ifm->ifi_index moved to
register_netdevice() from rtnl_newlink().

Let me understand finally: why do I need to reverse and move check
for dev->ifindex from register_netdevice() to rtnl_newlink()? That
will partially revert commit I note above.