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

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 07/14/2017 07:41 PM, Eric W. Biederman wrote:
> Stefan Berger <"Stefan Bergerstefanb"@linux.vnet.ibm.com> writes:
>
>> From: Stefan Berger <stefanb@linux.vnet.ibm.com>
>>
>> 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@linux.vnet.ibm.com>
>> Signed-off-by: Serge Hallyn <serge@hallyn.com>
>> Reviewed-by: Serge Hallyn <serge@hallyn.com>
>> ---
>>   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.

EVM could be converted to get the list of xattrs and prefix-compare it 
against the evm_config_xattrnames to do what it does now.

    Stefan