linux-kernel - Re: [PATCH 0/7] Initial support for user namespace owned mounts

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Date:	Thu, 16 Jul 2015 10:59:05 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 0/7] Initial support for user namespace owned mounts

On Thu, Jul 16, 2015 at 08:59:47AM -0500, Seth Forshee wrote:
> On Wed, Jul 15, 2015 at 10:15:21PM -0500, Eric W. Biederman wrote:
> > 
> > Seth I think for the LSMs we should start with:
> > 
> > diff --git a/security/security.c b/security/security.c
> > index 062f3c997fdc..5b6ece92a8e5 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -310,6 +310,8 @@ int security_sb_statfs(struct dentry *dentry)
> >  int security_sb_mount(const char *dev_name, struct path *path,
> >                         const char *type, unsigned long flags, void *data)
> >  {
> > +       if (current_user_ns() != &init_user_ns)
> > +               return -EPERM;
> >         return call_int_hook(sb_mount, 0, dev_name, path, type, flags, data);
> >  }
> 
> This just makes it impossible to mount from a user namespace. Every
> mount from current_user_ns() != &init_user_ns will fail.

What might work instead is to add a check in security_sb_kern_mount.
Then it would need to check s_user_ns, that way if proc, sysfs, etc.
use sget_userns(..., &init_user_ns) they can still be mounted in
containers.

It would be nicer to have a hook after sget but before fill_super so
that a bunch of work doesn't have to be done and then undone. Right now
there doesn't seem to be any suitable hook.

> > Then we should push this down into all of the lsms.
> > Then when we should remove or relax or change the check as appropriate
> > in each lsm.
> > 
> > The point is this is good enough to see that it is trivially safe,
> > and this allows us to focus on the core issues, and stop worrying about
> > the lsms for a bit.
> > 
> > Then we can focus on each lsm one at at time and take the time to really
> > understand them and talk with their maintainers etc to make certain
> > we get things correct.
> > 
> > This should remove the need for your patches 5, 6 and 7. For the
> > immediate future.
> 
> I'm still not entirely sure what you were trying to do, maybe refuse to
> mount whenever a security module is loaded? I think this could be a good
> option to start, but couldn't we restrict it to only the LSMs which use
> xattrs for security labels? In situations where the filesystem cannot
> supply security policy metadata I can't think of any reason to disallow
> the mounts.
> 
> Seth
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