Re: [kernel-hardening] Re: [PATCH resend 2/2] userns: control capabilities of some user namespaces

[Mahesh Bandewar (àààà ààààààà)]

[resend response as earlier one failed because of formatting issues]

On Thu, Nov 9, 2017 at 12:21 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
>
> On Thu, Nov 09, 2017 at 09:55:41AM +0900, Mahesh Bandewar (àààà ààààààà) wrote:
> > On Thu, Nov 9, 2017 at 4:02 AM, Christian Brauner
> > <christian.brauner@xxxxxxxxxxxxx> wrote:
> > > On Wed, Nov 08, 2017 at 03:09:59AM -0800, Mahesh Bandewar (àààà ààààààà) wrote:
> > >> Sorry folks I was traveling and seems like lot happened on this thread. :p
> > >>
> > >> I will try to response few of these comments selectively -
> > >>
> > >> > The thing that makes me hesitate with this set is that it is a
> > >> > permanent new feature to address what (I hope) is a temporary
> > >> > problem.
> > >> I agree this is permanent new feature but it's not solving a temporary
> > >> problem. It's impossible to assess what and when new vulnerability
> > >> that could show up. I think Daniel summed it up appropriately in his
> > >> response
> > >>
> > >> > Seems like there are two naive ways to do it, the first being to just
> > >> > look at all code under ns_capable() plus code called from there.  It
> > >> > seems like looking at the result of that could be fruitful.
> > >> This is really hard. The main issue that there were features designed
> > >> and developed before user-ns days with an assumption that unprivileged
> > >> users will never get certain capabilities which only root user gets.
> > >> Now that is not true anymore with user-ns creation with mapping root
> > >> for any process. Also at the same time blocking user-ns creation for
> > >> eveyone is a big-hammer which is not needed too. So it's not that easy
> > >> to just perform a code-walk-though and correct those decisions now.
> > >>
> > >> > It seems to me that the existing control in
> > >> > /proc/sys/kernel/unprivileged_userns_clone might be the better duct tape
> > >> > in that case.
> > >> This solution is essentially blocking unprivileged users from using
> > >> the user-namespaces entirely. This is not really a solution that can
> > >> work. The solution that this patch-set adds allows unprivileged users
> > >> to create user-namespaces. Actually the proposed solution is more
> > >> fine-grained approach than the unprivileged_userns_clone solution
> > >> since you can selectively block capabilities rather than completely
> > >> blocking the functionality.
> > >
> > > I've been talking to StÃphane today about this and we should also keep in mind
> > > that we have:
> > >
> > > chb@conventiont|~
> > >> ls -al /proc/sys/user/
> > > total 0
> > > dr-xr-xr-x 1 root root 0 Nov  6 23:32 .
> > > dr-xr-xr-x 1 root root 0 Nov  2 22:13 ..
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_cgroup_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_inotify_instances
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_inotify_watches
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_ipc_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_mnt_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_net_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_pid_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_user_namespaces
> > > -rw-r--r-- 1 root root 0 Nov  8 19:48 max_uts_namespaces
> > >
> > > These files allow you to limit the number of namespaces that can be created
> > > *per namespace* type. So let's say your system runs a bunch of user namespaces
> > > you can do:
> > >
> > > chb@conventiont|~
> > >> echo 0 > /proc/sys/user/max_user_namespaces
> > >
> > > So that the next time you try to create a user namespaces you'd see:
> > >
> > > chb@conventiont|~
> > >> unshare -U
> > > unshare: unshare failed: No space left on device
> > >
> > > So there's not even a need to upstream a new sysctl since we have ways of
> > > blocking this.
> > >
> > I'm not sure how it's solving the problem that my patch-set is addressing?
> > I agree though that the need for unprivileged_userns_clone sysctl goes
> > away as this is equivalent to setting that sysctl to 0 as you have
> > described above.
>
> oh right that was the reasoning iirc for not needing the other sysctl.
>
> > However as I mentioned earlier, blocking processes from creating
> > user-namespaces is not the solution. Processes should be able to
> > create namespaces as they are designed but at the same time we need to
> > have controls to 'contain' them if a need arise. Setting max_no to 0
> > is not the solution that I'm looking for since it doesn't solve the
> > problem.
>
> well yesterday we were told that was explicitly not the goal, but that was
> not by you ... i just mention it to explain why we seem to be walking in
> circles a bit.
>
> anyway the bounding set doesn't actually make sense so forget that.   the
> question then is just whether it makes sense to allow things to continue
> at all in this situation.  would you mind indulging me by giving one or two
> concrete examples in the previous known cves of what capabilities you would
> have dropped tto allow the rest to continue to be safely used?
>
Of course. Let's take an example of the CVE that I have mentioned in
my cover-letter -
CVE-2017-7308(https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-7308).
It's well documented and even has a
exploit(https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-7308)
c-program that can demonstrate how it can be used against non-patched
kernel. There is very nice blog
post(https://googleprojectzero.blogspot.kr/2017/05/exploiting-linux-kernel-via-packet.html)
about this vulnerability by Andrey Konovalov.

This is about the AF_PACKET socket interface that is protected behind
NET_RAW capability. This capability is not available to unprivileged
user. However, any unprivileged user can get NET_RAW capability (as
demonstrated in the cover-letter code that I have attached in this
patch series) so this NET_RAW capability is available to any
unprivileged user on the host if the kernel has user-namespaces
available.

With this patch-set applied, all that is needed is to flip a bit with
the sysctl (kernel.controlled_userns_caps_whitelist) as demonstrated
below -

root@lphh6:~# uname -a
Linux lphh6 4.14.0-smp-DEV #97 SMP @1510203579 x86_64 GNU/Linux
root@lphh6:~# sysctl -q kernel.controlled_userns_caps_whitelist
kernel.controlled_userns_caps_whitelist = 1f,ffffffff

Now when I run the program (demo from the cover-letter) as a normal
unprivileged user I can't create a RAW socket in init-ns but I can in
the child-ns.

dumbo@lphh6:~$ /tmp/acquire_raw
Attempting to open RAW socket before unshare()...
socket() SOCK_RAW failed: : Operation not permitted
Attempting to open RAW socket after unshare()...
Successfully opened RAW-Sock after unshare().
dumbo@lphh6:~$

Now as a root user. Take off CAP_NET_RAW

root@lphh6:~# sysctl -w kernel.controlled_userns_caps_whitelist=1f,ffffdfff
kernel.controlled_userns_caps_whitelist = 1f,ffffdfff
root@lphh6:~#

Now run the same program as an unprivileged user -

dumbo@lphh6:~$ /tmp/acquire_raw
Attempting to open RAW socket before unshare()...
socket() SOCK_RAW failed: : Operation not permitted
Attempting to open RAW socket after unshare()...
socket() SOCK_RAW failed: : Operation not permitted
dumbo@lphh6:~$

Notice that it has failed to create a raw socket in init and in child
namespace. It's not blocking creation of user-namespaces but allowing
admin turn individual capability bits on and off.

This is very simplistic example of just demonstrating how capability
bits turn-on/off works. So let's assume a sandboxed environment where
we don't know what a binary that we are about run in an environment
which is identified as susceptible. By turning off the NET_RAW bit,
the admin gets an assurance that system is safe and if binary fails
because it's not getting this capability then that bad but a sad
consequence (without compromising the host integrity) but if it
doesn't use the NET_RAW capability but any other combination of
remaining 36 capabilities, it would get whatever is necessary. This
means we can safely allow processes to create user-namespaces by
taking off certain capabilities in question for temporary/extended
period until proper fix is applied without compromising the system
integrity. The impact will vary based on which capability is taken off
and admin would / should be ware of for the environment that he/she is
dealing with.

thanks,
--mahesh..

> thanks,
> serge