Re: CVE-2020-16120 - incorrect unprivileged overlayfs permission checking

[Jordan Glover <Golden_Miller83 () protonmail ch>]

On Tuesday, October 13, 2020 5:10 PM, Steve Beattie <steve.beattie () canonical com> wrote:

> Hello,
>
> CVE-2020-16120 - incorrect unprivileged overlayfs permission checking
>
> Giuseppe Scrivano discovered that overlayfs did not properly perform
> permission checking when copying up files in an overlayfs, and can be
> exploited from within a user namespace, if, for example, unprivileged
> user namespaces are allowed.
>
> An attacker can abuse this to get read access to files on the system
> that they would not normally be permitted to access.
>
> This likely only has an impact on Ubuntu kernels, where unprivileged
> user namespaces are enabled by default.

AFAIK unpriv user ns are enabled by default on vast majority of distros nowadays with debian (rhel?) being an exception 
(although this is going to change at some point[1]). I think what makes ubuntu different is unpriv overlayfs which 
doesn't exist upstream thus in most other distros.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=898446

> The following upstream commits address the issue:
>
> 48bd024b8a40d73ad6b086de2615738da0c7004f ("ovl: switch to mounter creds in readdir")
> 56230d956739b9cb1cbde439d76227d77979a04d ("ovl: verify permissions in ovl_path_open()")
> 05acefb4872dae89e772729efb194af754c877e8 ("ovl: check permission to open real file")
>
> The following commits also may be desired or necessary:
>
> 130fdbc3d1f9966dd4230709c30f3768bccd3065 ("ovl: pass correct flags for opening real directory")
> 292f902a40c11f043a5ca1305a114da0e523eaa3 ("ovl: call secutiry hook in ovl_real_ioctl()")
>
> Mitigation on systems where unprivileged user namespaces are enabled
> but not needed is to set the kernel.unprivileged_userns_clone sysctl
> to 0. e.g.:
>
> $ sudo sysctl kernel.unprivileged_userns_clone=0
>
> and across reboots by adding a file in /etc/sysctl.d/ that contains:
>
> kernel.unprivileged_userns_clone=0

This will only work with out-of-tree patch included in distro kernel.

> Thanks.
>
>
>
> Steve Beattie
> sbeattie () ubuntu com