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

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Date:	Fri, 22 Jan 2016 13:46:32 -0800
From:	Kees Cook <keescook@...gle.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	David Howells <dhowells@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	syzkaller <syzkaller@...glegroups.com>,
	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>
Subject: Re: fs: sandboxed process brings host down

On Fri, Jan 22, 2016 at 1:21 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Jan 22, 2016 at 10:06:14PM +0100, Dmitry Vyukov wrote:
>> Hello,
>>
>> While running syzkaller fuzzer I hit the following problem. Supervisor
>> process sandboxes worker processes that do random activities with
>> CLONE_NEWUSER | CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWUTS |
>> CLONE_NEWNET | CLONE_NEWIPC | CLONE_IO, setrlimit, chroot, etc.
>> Because of that worker process gains ability to bring whole machine
>> down (does not happen without the sandbox).
>
> AFAICS, what you are doing is essentially mount --rbind / / in infinite
> loop in luserns.  Which ends up eating all memory.  There's any number
> of ways to do the same.  We can play whack-a-mole with them until the
> kernel is completely ossified with accounting code of different sorts.
> Or one can disable userns and be done with that.

I think it's time for a runtime tunable to disable userns. For all the
distro users that are forced to run with kernels built with
CONFIG_USER_NS, there's no way for them to escape these USER_NS flaws
when they don't use the feature.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security