Re: [PATCH v3.4] capabilities: require CAP_SETFCAP to map uid 0

[Christian Brauner]

On Tue, Apr 20, 2021 at 08:43:34AM -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:
>     https://git.kernel.org/pub/scm/linux/kernel/git/sergeh/linux.git/log/?h=2021-04-15/setfcap-nsfscaps-v4
> 
> History:
> 
> Commit 95ebabde382 ("capabilities: Don't allow writing ambiguous v3 file
> capabilities") tried to fix the issue by preventing v3 fscaps to be
> written to disk when the root uid would map to the same uid in nested
> user namespaces. This led to regressions for various workloads. For
> example, see [1]. Ultimately this is a valid use-case we have to support
> meaning we had to revert this change in 3b0c2d3eaa83 ("Revert
> 95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file
> capabilities")").
> 
> [1]: https://github.com/containers/buildah/issues/3071
> 
> 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>
> Tested-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

If there's no objections then Linus can probably just pick up the single
patch here directly:
https://lore.kernel.org/lkml/20210420134334.GA11582@xxxxxxxxxxxxxxx

I'm not sure it's worth waiting and releasing another kernel with this
bug. This tigthens the semantics nicely and makes for a simple check at
userns creation time instead of repeatedly checking at setxattr(). With
all the testing done we can be quite confident the risk of regressions
is way lower than the old patch and even if we see one I think this
version of the fix is actually worth the risk.

Christian