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Re: [PATCH v4] Introduce v3 namespaced file capabilities




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

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()) 6) security.capability@* are 'reserved' and may be read but not written to unless 5) applies.


Similat, from the text of one of the functions in the code:

+ * In a user namespace we prevent read/write accesses to the _host's_
+ * security.foo to protect these extended attributes.
+ *
+ * Reading: Reading security.foo from a user namespace will read
+ * security.foo@uid=<uid> instead. Reading security.foo@uid=<uid> directly
+ * also works. In general, all security.foo*, except for security.foo of the
+ * host, can be read from a user namespace.
+ *
+ * Writing: Writing security.foo from a user namespace will write
+ * security.foo@uid=<uid> instead. Writing security.foo@uid=<uid> directly
+ * also work.s No other security.foo* attributes, including the security.foo
+ * offthe host, can be written to. All security.foo@* are 'reserved'.
+ *
+ * Removing: The same rules for writing apply to removing of extended
+ * attributes from a user namespace.



   Stefan