Re: [PATCH 0/3] Enable namespaced file capabilities

[Vivek Goyal]

On Fri, Jun 23, 2017 at 03:17:23PM -0500, Serge E. Hallyn wrote:
> Quoting Vivek Goyal (vgoyal@xxxxxxxxxx):
> > On Thu, Jun 22, 2017 at 02:59:46PM -0400, Stefan Berger wrote:
> > > This series of patches primary goal is to enable file capabilities
> > > in user namespaces without affecting the file capabilities that are
> > > effective on the host. This is to prevent that any unprivileged user
> > > on the host maps his own uid to root in a private namespace, writes
> > > the xattr, and executes the file with privilege on the host.
> > > 
> > > We achieve this goal by writing extended attributes with a different
> > > name when a user namespace is used. If for example the root user
> > > in a user namespace writes the security.capability xattr, the name
> > > of the xattr that is actually written is encoded as
> > > security.capability@uid=1000 for root mapped to uid 1000 on the host.
> > > When listing the xattrs on the host, the existing security.capability
> > > as well as the security.capability@uid=1000 will be shown. Inside the
> > > namespace only 'security.capability', with the value of
> > > security.capability@uid=1000, is visible.
> > 
> > Hi Stefan,
> > 
> > Got a question. If child usernamespace sets a
> > security.capability@uid=1000, can any of the parent namespace remove it?
> > 
> > IOW, I set capability from usernamespace and tried to remove it from
> > host and that failed. Is that expected.
> > 
> > # Inside usernamespce
> > $setcap cat_net_raw+ep foo.txt
> > 
> > # outside user namespace
> > $listxattr foo.txt
> >  xattr: security.capability@uid=1000
> >  xattr: security.selinux
> > 
> > # outside user namespace
> > setfattr -x security.capability@uid foo.txt
> > setfattr: foo.txt: Invalid argument
> > 
> > Doing a strace shows removexattr() failed. May this will need fixing?
> > 
> > removexattr("testfile.txt", "security.capability@uid") = -1 EINVAL
> > (Invalid argument)
> 
> That's not the right xattr, though, does
> 
> 	setfattr -x security.capability@uid=1000 foo.txt
> 
> work?

Yep, that works (as root on host). My bad.

> 
> If you are in fact uid=1000 then that should work.

Tried setfattr -x as uid 1000 in init_user_ns and that seems to have
issues.

$ ll testfile.txt 
-rw-r--r--. 1 vivek vivek 0 Jun 23 15:44 testfile.txt

$listxattr testfile.txt
xattr: security.capability@uid=1000
xattr: security.selinux

$id
uid=1000(vivek) gid=1000(vivek) groups=1000(vivek)
context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

$setfattr -x security.capability@uid=1000 testfile.txt 
setfattr: testfile.txt: Operation not permitted

I had to launch a user namespace with 1000 mapped to 0 inside user
namespace and then "setfattr -x security.capability testfile.txt" worked.

> If you are uid 1001,
> and 1000 was delegated to you, then you'll need to create a transient
> userns with uid 1000 mapped into it in order to delete it (so that you
> have privilege over the uid).

Will give this a try.

Vivek
> 
> If that doesn't work, then it's a bug.
> 
> -serge