Re: [PATCH v4] Introduce v3 namespaced file capabilities

From: Stefan Berger
Date: Mon Jun 19 2017 - 09:06:06 EST

On 06/18/2017 09:13 PM, Stefan Berger wrote:
On 06/18/2017 06:14 PM, Serge E. Hallyn wrote:
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 security.foo[130000] first, then
security.foo[100000], then security.foo. 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:

https://github.com/stefanberger/linux/commits/xattr_for_userns
Thanks!

Encoding of uid is in the attribute name now as follows:
security.foo@uid=<uid>

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.

I'll try to write a patch on top of the existing one.

Did that now in a 2nd patch (that also fixes a few problems of the 1st). In a user ns mapped to 1000 root can write security.capability@uid=123, which then ends up writing to security.capability@uid=1123. The reading also works with @uid=123. When listing xattrs only those get shown that actually have valid mappings.

https://github.com/stefanberger/linux/commits/xattr_for_userns

Stefan