linux-kernel - Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user

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Date:	Wed, 19 Nov 2014 11:38:14 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Andy Lutomirski <luto@...capital.net>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns

On Wed, Nov 19, 2014 at 9:50 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, Nov 18, 2014 at 4:21 PM, Seth Forshee
> <seth.forshee@...onical.com> wrote:
>>> I asked around a bit, and it turns out there are use cases for nested
>> containers (i.e. a container within a container) where the rootfs for
>> the outer container mounts a filesystem containing the rootfs for the
>> inner container. If that mount is nosuid then suid utilities like ping
>> aren't going to work in the inner container.
>>
>> So since there's a use case for suid in a userns mount and we have what
>> we belive are sufficient protections against using this as a vector to
>> get privileges outside the container, I'm planning to move ahead without
>> the MNT_NOSUID restriction. Any objections?
>
> In the general case how'd we prevent suid executable being tricked to
> do something it shouldn't do by unprivileged mounting into sensitive
> places (i.e. config files) inside the container?
>
> Allowing SUID looks like a slippery slope to me.  And there are plenty
> of solutions to the "ping" problem, AFAICS, that don't involve the
> suid bit.

ping isn't even suid on my system, it has security.capability xattr instead.

Please just get rid of SUID/SGID.  It's a legacy, it's a hack, not
worth the complexity and potential problems arising from that
complexity.

Thanks,
Miklos
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