linux-kernel - Re: Connecting to sockets on MNT

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Message-ID: <CALCETrXhVKq33-KWLMVxPtCZgHddKsxCOQX9F-LGerXsCrLH5Q@mail.gmail.com>
Date:	Thu, 1 May 2014 16:57:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: Connecting to sockets on MNT_READONLY mounts?

On Thu, May 1, 2014 at 4:51 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, May 01, 2014 at 04:00:49PM -0700, Andy Lutomirski wrote:
>> On Thu, May 1, 2014 at 3:34 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> > On Thu, May 01, 2014 at 03:20:00PM -0700, Andy Lutomirski wrote:
>> >> Is it supposed to work?
>> >
>> > Why the hell not?  Same as opening a device node on r/o filesystem for
>> > write, or doing the same with FIFO.
>>
>> You can't bind a socket on a read-only fs, so I thought it was a fair question.
>>
>> I'll write a patch to add MS_NOIPCCONNECT and MNT_NOIPCCONNECT to
>> block connect on unix sockets and open on fifos.  This will be useful
>> for sandboxes that want to prevent sandboxed programs from accessing
>> undesirable parts of the outside world.
>
> Sigh...  Don't expose those FIFOs et.al. to them, then.
> mount --bind /dev/null <pathname>
> as part of setting the sucker up.  And if you *are* blindly exposing the
> host filesystems to them wholesale, sockets and fifos are the least of your
> problems, even if you do that read-only.

Why?

Suppose I bind-mount /usr into a private namespace with
nosuid,nodev,ro.  How can you use it to attack anything?  The only
thing I've thought of is to open fifos and connect to sockets.  I'm
assuming that there's a pid namespace blocking ptrace and such and a
network namespace blocking abstract sockets.

--Andy
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