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SubjectRe: [PATCH review 5/6] userns: Allow the userns root to mount ramfs.
Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serge@hallyn.com> writes:
>
> > Quoting Eric W. Biederman (ebiederm@xmission.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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