linux-kernel - Re: fs: sandboxed process brings host down

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Date:	Fri, 22 Jan 2016 23:32:00 +0100
From:	Robert Święcki <robert@...ecki.net>
To:	syzkaller@...glegroups.com
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	David Howells <dhowells@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Kees Cook <keescook@...gle.com>
Subject: Re: fs: sandboxed process brings host down

2016-01-22 22:55 GMT+01:00 Al Viro <viro@...iv.linux.org.uk>:
> On Fri, Jan 22, 2016 at 10:38:40PM +0100, Dmitry Vyukov wrote:
>
>> My 2GB VM dies at around just 10-th iteration, is it normal?
>> Each iteration consumes several hundreds of megs of kernel memory. And
>> there seems to be exponential slowdown at around 5-th iteration.
>> I understand that there can be lots of forms of a local DoS. But there
>> seems to be something pathological about this particular one. And it
>> happens only with sandboxing that is meant to reduce DoS
>> possibilities...
>
> Sandboxing == giving attacker to do mount without being root.

I was discussing this initially with Dmitry and maybe I explained it
initially a bit incorrectly. I did not mean to suggest that using
CLONE_NEWUSER alone is a form of sandboxing. But rather, that when
used correctly (with dropping capabilities, rlimits and seccomp-bpf
filters) it could constitute a form of sandboxing.

What I suggested was to use CLONE_NEWUSER and friends to test the new
attack surface, which is enabled by using CLONE_NEW*. For regular
users the syscall(__NR_mount) returns early with EPERM, but when
CLONE_NEW* are used, a new, big attack surface opens up, reachable
from a level of a unprivileged user.

So, I guess, it's not about sandboxing but the newly reachable attack surface.

-- 
Robert Święcki