Re: [PATCH v4] Introduce v3 namespaced file capabilities

[Stefan Berger]

On 06/20/2017 01:42 AM, Amir Goldstein wrote:
> On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
> > "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 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.
> > >
> > > 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.
> Apropos stackable filesystems [cc some overlayfs folks], is there any
> way that parts of this work could be generalized towards ns aware
> trusted@uid.* xattr?

I am at least removing all string comparison with xattr names from the 
core code and move the enabled xattr names into a list. For the 
security.* extended attribute names we would enumerated the enabled ones 
in that list, only security.capability for now. I am not sure how the 
trusted.* space works.

    Stefan

> With overlayfs, files are written to underlying fs with mounter's
> credentials. How this affects v3 security capabilities and how exactly
> security xattrs are handled in overtlayfs I'm not sure. Vivek?
>
> But, if we had an infrastructure to store trusted@<rootid> xattr, then
> unprivileged overlayfs mount would become a very reachable goal.
> Much closer goal then loop mounting...
>
> Amir.