Re: [PATCH v2] xattr: Enable security.capability in user namespaces

From: Eric W. Biederman
Date: Fri Jul 14 2017 - 19:50:19 EST


Stefan Berger <"Stefan Bergerstefanb"@linux.vnet.ibm.com> writes:

> From: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
>
> This patch enables security.capability in user namespaces but also
> takes a more general approach to enabling extended attributes in user
> namespaces.
>
> The following rules describe the approach using security.foo as a
> 'user namespace enabled' extended attribute:
>
> Reading of extended attributes:
>
> 1a) Reading security.foo from a user namespace will read
> security.foo@uid=<uid> of the parent user namespace instead with uid
> being the mapping of root in that parent user namespace. An
> exception is if root is mapped to uid 0 on the host, and in this case
> we will read security.foo directly.
> --> reading security.foo will read security.foo@uid=1000 for uid
> mapping of root to 1000.
>
> 1b) If security.foo@uid=<uid> is not available, the security.foo of the
> parent namespace is tried to be read. This procedure is repeated up to
> the init user namespace. This step only applies for reading of extended
> attributes and provides the same behavior as older system where the
> host's extended attributes applied to user namespaces.
>
> 2) All security.foo@uid=<uid> with valid uid mapping in the user namespace
> can be read. The uid within the user namespace will be mapped to the
> corresponding uid on the host and that uid will be used in the name of
> the extended attribute.
> -> reading security.foo@uid=1 will read security.foo@uid=1001 for uid
> mapping of root to 1000, size of at least 2.
>
> All security.foo@uid=<uid> can be read (by root) on the host with values
> of <uid> also being subject to checking for valid mappings.
>
> 3) No other security.foo* can be read.
>
> The same rules for reading apply to writing and removing of user
> namespace enabled extended attributes.
>
> When listing extended attributes of a file, only those are presented
> to the user namespace that have a valid mapping. Besides that, names
> of the extended attributes are adjusted to represent the mapping.
> This means that if root is mapped to uid 1000 on the host, the
> security.foo@uid=1000 will be listed as security.foo in the user
> namespace, security.foo@uid=1001 becomes security.foo@uid=1 and so on.
>
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx>
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
> ---
> fs/xattr.c | 509 +++++++++++++++++++++++++++++++++++++++++++++--
> security/commoncap.c | 36 +++-
> security/selinux/hooks.c | 9 +-
> 3 files changed, 523 insertions(+), 31 deletions(-)

I am just going to quickly and publicly point out that as designed this
patch breaks evm inode metadata signing. As evm_config_xattrnames is not
updated.

While not completely insurmountable that seems like a strong limitation of
this design.

Eric