Re: [PATCH] capabilities: require CAP_SETFCAP to map uid 0 (v3.3)

From: Christian Brauner
Date: Mon Apr 19 2021 - 12:09:24 EST

On Mon, Apr 19, 2021 at 07:25:14AM -0500, Serge Hallyn wrote:
> cap_setfcap is required to create file capabilities.
> Since 8db6c34f1dbc ("Introduce v3 namespaced file capabilities"), a
> process running as uid 0 but without cap_setfcap is able to work around
> this as follows: unshare a new user namespace which maps parent uid 0
> into the child namespace. While this task will not have new
> capabilities against the parent namespace, there is a loophole due to
> the way namespaced file capabilities are represented as xattrs. File
> capabilities valid in userns 1 are distinguished from file capabilities
> valid in userns 2 by the kuid which underlies uid 0. Therefore the
> restricted root process can unshare a new self-mapping namespace, add a
> namespaced file capability onto a file, then use that file capability in
> the parent namespace.
> To prevent that, do not allow mapping parent uid 0 if the process which
> opened the uid_map file does not have CAP_SETFCAP, which is the capability
> for setting file capabilities.
> As a further wrinkle: a task can unshare its user namespace, then
> open its uid_map file itself, and map (only) its own uid. In this
> case we do not have the credential from before unshare, which was
> potentially more restricted. So, when creating a user namespace, we
> record whether the creator had CAP_SETFCAP. Then we can use that
> during map_write().
> With this patch:
> 1. Unprivileged user can still unshare -Ur
> ubuntu@caps:~$ unshare -Ur
> root@caps:~# logout
> 2. Root user can still unshare -Ur
> ubuntu@caps:~$ sudo bash
> root@caps:/home/ubuntu# unshare -Ur
> root@caps:/home/ubuntu# logout
> 3. Root user without CAP_SETFCAP cannot unshare -Ur:
> root@caps:/home/ubuntu# /sbin/capsh --drop=cap_setfcap --
> root@caps:/home/ubuntu# /sbin/setcap cap_setfcap=p /sbin/setcap
> unable to set CAP_SETFCAP effective capability: Operation not permitted
> root@caps:/home/ubuntu# unshare -Ur
> unshare: write failed /proc/self/uid_map: Operation not permitted
> Note: an alternative solution would be to allow uid 0 mappings by
> processes without CAP_SETFCAP, but to prevent such a namespace from
> writing any file capabilities. This approach can be seen here:

Ah, can you link to the previous fix and its revert, please? I think
that was mentioned in the formerly private thread as well but we forgot:

commit 95ebabde382c371572297915b104e55403674e73
Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Date: Thu Dec 17 09:42:00 2020 -0600

capabilities: Don't allow writing ambiguous v3 file capabilities

commit 3b0c2d3eaa83da259d7726192cf55a137769012f
Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Date: Fri Mar 12 15:07:09 2021 -0600

Revert 95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file capabilities")

> Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx>
> Reviewed-by: Andrew G. Morgan <morgan@xxxxxxxxxx>
> Tested-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> Reviewed-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>