netdev - Re: [PATCH] net:ppp: replace too strict capability restriction on opening /dev/ppp

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Message-ID: <57679170.5030406@nod.at>
Date:	Mon, 20 Jun 2016 08:47:12 +0200
From:	Richard Weinberger <richard@....at>
To:	Andi Kleen <andi@...stfloor.org>,
	Shanker Wang <shanker@...a.tsinghua.edu.cn>
Cc:	netdev@...r.kernel.org,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Guillaume Nault <g.nault@...halink.fr>,
	Miao Wang <shankerwangmiao@...il.com>
Subject: Re: [PATCH] net:ppp: replace too strict capability restriction on
 opening /dev/ppp

Am 20.06.2016 um 07:02 schrieb Andi Kleen:
> Shanker Wang <shanker@...a.tsinghua.edu.cn> writes:
> 
>> This patch removes the check for CAP_NET_ADMIN in the initial namespace
>> when opening /dev/open. Instead, CAP_NET_ADMIN is checked in the user
>> namespace the net namespace was created so that /dev/ppp cat get opened
>> in a unprivileged container.
> 
> Seems dangerous. From a quick look at the PPP ioctl there is no limit
> how many PPP devices this can create. So a container having access to
> this would be able to fill all kernel memory. Probably needs more
> auditing and hardening first.
> 
> In general there seems to be a lot of attack surface for root
> in PPP.

You are right.
Shanker Wang, I had also another at the open function, it is more complicated
than I thought. Please see how ppp_unattached_ioctl() is called.
Before we give containers access to it the use of nsproxy has to be removed.
Not sure how easy this will be, especially since you cannot break existing users.

Thanks,
//richard