linux-kernel - Re: [PATCH review 5/6] userns: Allow the userns root to mount ramfs.

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Date:	Sun, 27 Jan 2013 18:23:21 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH review 5/6] userns: Allow the userns root to mount
 ramfs.

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serge@...lyn.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm@...ssion.com):
> >> 
> >> There is no backing store to ramfs and file creation
> >> rules are the same as for any other filesystem so
> >> it is semantically safe to allow unprivileged users
> >> to mount it.
> >> 
> >> The memory control group successfully limits how much
> >> memory ramfs can consume on any system that cares about
> >> a user namespace root using ramfs to exhaust memory
> >> the memory control group can be deployed.
> >
> > But that does mean that to avoid this new type of attack, when handed a
> > new kernel (i.e. by one's distro) one has to explicitly (know about and)
> > configure those limits.  The "your distro should do this for you"
> > argument doesn't seem right.  And I'd really prefer there not be
> > barriers to user namespaces being compiled in when there don't have to
> > be.
> 
> The thing is this really isn't a new type of attack.  There are a lot of

Of course.

> existing methods to exhaust memory with the default configuration on
> most distros.  All this is is a new method to method to implement such
> an attack.

Right.

...

> > What was your thought on the suggestion to only allow FS_USERNS_MOUNT
> > mounts by users confined in a non-init memory cgroup?
> 
> Over design.

Ok.  Fair.

> So shrug.  The mechanisms that I am suggesting people use already exist,
> and appear to have been present long enough to have made it into debian
> stable release February of 2011.

Heh - right, libcgroup does have its problems, but I don't think there
are any problems with the pam module actually.  I'm meant to talk with
the debian maintainer for them soon, will test.

thanks,
-serge
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