[kernel-hardening] Re: [PATCH/RFC] user_ns: fix missing limiting of user_ns counts

[Vasily Kulikov <segoon@openwall.com>]

On Fri, Dec 28, 2012 at 20:05 -0800, Eric W. Biederman wrote:
> Vasily Kulikov <segoon@openwall.com> writes:
> 
> > Currently there is completely no limiting in number of user namespaces
> > created by unprivileged users.  One can freely create thousands of
> > user_ns'es and exhaust kernel memory without even bumping in
> > RLIMIT_NPROC or similar.
> 
> First for a proper sense of scale it will take roughly 14,000 to consume
> a megabyte.  So it will take hundreds of millions of user namespaces to
> eat up all of kernel memory.

Yes, but you can freely create *any* number of nested userns by a loop:

    for() {
        unshare()
        write to /proc/self/{u,g}id_map
    }

> > The code needs several checks.  First, noone should be able to create
> > user_ns of arbitrary depth.  Besides kernel stack overflow one could
> > create too big depth to DoS processes belonging to other users by
> > forcing them to loop a long time in cap_capable called from some
> > ns_capable() (e.g. in case one does smth like "ls -R /proc").
> 
> Where do you get a ns_capable call from "ls -R /proc" ?

E.g. if procfs is mounted with hidepid=2 then ls does
ptrace_may_access() check. 

Thanks,

-- 
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments