Re: [PATCH v4] Introduce v3 namespaced file capabilities

From: Eric W. Biederman
Date: Mon Jun 19 2017 - 17:42:18 EST

"Serge E. Hallyn" <serge@xxxxxxxxxx> writes:

> Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>> >On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>> >>On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>> >>>Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
>> >>>> If all extended
>> >>>>attributes were to support this model, maybe the 'uid' could be
>> >>>>associated with the 'name' of the xattr rather than its 'value' (not
>> >>>>sure whether that's possible).
>> >>>Right, I missed that in your original email when I saw it this morning.
>> >>>It's not what my patch does, but it's an interesting idea. Do you have
>> >>>a patch to that effect? We might even be able to generalize that to
>> >>No, I don't have a patch. It may not be possible to implement it.
>> >>The xattr_handler's take the name of the xattr as input to get().
>> >That may be ok though. Assume the host created a container with
>> >100000 as the uid for root, which created a container with 130000 as
>> >uid for root. If root in the nested container tries to read the
>> >xattr, the kernel can check for[130000] first, then
>> >[100000], then Or, it can do a listxattr
>> >and look for those. Am I overlooking one?
>> >
>> >>So one could try to encode the mapped uid in the name. However, that
>> >I thought that's exactly what you were suggesting in your original
>> >email? "security.capability[uid=2000]"
>> >
>> >>could lead to problems with stale xattrs in a shared filesystem over
>> >>time unless one could limit the number of xattrs with the same
>> >>prefix, e.g., security.capability*. So I doubt that it would work.
>> >Hm. Yeah. But really how many setups are there like that? I.e. if
>> >you launch a regular docker or lxd container, the image doesn't do a
>> >bind mount of a shared image, it layers something above it or does a
>> >copy. What setups do you know of where multiple containers in different
>> >user namespaces mount the same filesystem shared and writeable?
>> I think I have something now that accomodates userns access to
>> security.capability:
> Thanks!
>> Encoding of uid is in the attribute name now as follows:
>> 1) The 'plain' security.capability is only r/w accessible from the
>> host (init_user_ns).
>> 2) When userns reads/writes 'security.capability' it will read/write
>> security.capability@uid=<uid> instead, with uid being the uid of
>> root , e.g. 1000.
>> 3) When listing xattrs for userns the host's security.capability is
>> filtered out to avoid read failures iof 'security.capability' if
>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>> 4) security.capability* may all be read from anywhere
>> 5) security.capability@uid=<uid> may be read or written directly
>> from a userns if <uid> matches the uid of root (current_uid())
> This looks very close to what we want. One exception - we do want
> to support root in a user namespace being able to write
> security.capability@uid=<x> where <x> is a valid uid mapped in its
> namespace. In that case the name should be rewritten to be
> security.capability@uid=<y> where y is the unmapped kuid.val.
> Eric,
> so far my patch hasn't yet hit Linus' tree. Given that, would you
> mind taking a look and seeing what you think of this approach? If
> we may decide to go this route, we probably should stop my patch
> from hitting Linus' tree before we have to continue supporting it.

Agreed. I will take a look. I also want to see how all of this works
in the context of stackable filesystems. As that is the one case that
looked like it could be a problem case in your current patchset.