PostUp/PreUp/PostDown/PreDown Dangerous?

[Dario Bosch]

Hi all,

I've been following this discussion and now also want to add my 2 cents.

In my opinion wireguard's biggest strength is the simplicity of its
config. One can write a 8-line config file that works reliably, is
secure and also can take care of ip tables rules, start services etc.
Even a user who only has a rough understanding about their system can
achieve this.
Everything that makes using wireguard more complicated should be
avoided. Setting up other VPNs such as openVPN is a hassle simply
because they are *not* designed to be simple.

We are discussing about an attack scenario, where the attacker convinces
a clueless user to compile/install a kernel module, we always should
keep that in mind. Most installation instructions advise people to add a
custom repository or clone a git repository, so they're already running
foreign, untrusted code with root rights.

In my opinion the only viable option would be to computer a check-sum
over the {Pre/Post}{Up/Down} lines in the config files i wg-quick. In
case the check-sum has changed, the user could then be asked to trust
this file. I imagine it a bit like that:

    [root at machine /]$ wg-quick up myWG
    [#] *New Post-Up rule detected*
    [#] Warning: The selected config file will execute the following
    [#] commands when loaded:
    [#]      'rm -rf /'
    [#] Do you want to trust this config file? [y]/[n]
       > y
    [#] Rule added to list of trusted config, continuing
    ...

Again, I think making wireguard usage more complicated and annoying than
necessary should be avoided.

Cheers,
Dario




On 06/23/2018 04:36 AM, Antonio Quartulli wrote:
> Hi,
> 
> On 23/06/18 06:13, Jordan Glover wrote:
> [cut]
>>
>> But attacker will helpfully provide you customized 'wireguard.script'  as well
>> and even tell you how to use it by setting 'chmod 4777 wireguard.script'.
>>
> 
> An attacker will also tell you to run "rm -Rf /" :-P
> 
> 
> Jokes apart, I was talking to Jason on IRC and I suggested an idea that
> might be worth sharing.
> 
> A network device driver in the kernel is free to send events to
> userspace with any custom set of properties/values.
> 
> Most of you have already seen and played with those typically thrown
> when an interface goes up and down, with udev normally handling them by
> executing some (user-)configured action.
> 
> These events can be easily created and customized by any kernel module
> and associated to a network interface.
> Wireguard could generate preup/postup/etc.. uevents and send them to
> userspace.
> 
> It will then be udev to decide how to handle those.
> Specific scripts could be installed by the admin, or udev could come
> with its own default ones.
> 
> In any case, this would delegate the execution of scripts to a component
> that is in charge of doing exactly that.
> 
> This would remove the risk of sneaking malicious things into the
> configuration file, which is what people do not expect and is the core
> of the issue discussed here.
> 
> (Yeah, I already hear people saying "but the malicious attacker will
> tell the clueless user to install this script in udev", but I think that
> by then, the problem has moved to another plane)
> 
> My experience with this mechanism comes from batman-adv[1], where it
> used to report special routing events to the user so that he could react
> accordingly (if desired).
> 
> 
> just my 2 cents.
> 
> 
> Cheers,
> 
> [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/batman-adv/sysfs.c#n1209
> 
> 
> 
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