[manpages PATCH] capabilities.7: describe namespaced file capabilities

Jann Horn jannh at google.com
Mon Apr 16 14:10:38 UTC 2018

On Fri, Apr 13, 2018 at 9:26 PM, Michael Kerrisk (man-pages)
<mtk.manpages at gmail.com> wrote:
> Hello Serge, Jann,
> On 01/16/2018 06:26 PM, Jann Horn wrote:
>> On Tue, Jan 9, 2018 at 7:52 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
>        Starting  with  Linux  4.14,  a  security.capability   extended
>        attribute  is automatically created as (or converted to) a ver‐
>        sion 3 (VFS_CAP_REVISION_3) attribute if both of the  following
>        are true:
>        (1) The  thread  writing  the attribute resides in a noninitial
>            namespace.

I'm not entirely happy with this - while under most circumstances
(especially nowadays) correct, isn't this going to confuse readers who
want to understand the actual rules?

>            (More precisely: the thread resides in  a  user
>            namespace  other  than  the  one  from which the underlying
>            filesystem was mounted.)

I think if you're in a parent namespace of the user namespace that
mounted the filesystem, you actually can write a VFS_CAP_REVISION_2

>        (2) The thread has the CAP_SETFCAP  capability  over  the  file
>            inode,  meaning  that  (a)  the  thread has the CAP_SETFCAP
>            capability in its own user namespace; and (b) the  UID  and
>            GID  of  the  file inode have mappings in the writer's user
>            namespace.

>            ┌─────────────────────────────────────────────────────┐
>            │FIXME                                                │
>            ├─────────────────────────────────────────────────────┤
>            │Does there also need to be some kind  of  credential │
>            │match  between  the  file  and the namespace creator │
>            │UID?                                                 │
>            └─────────────────────────────────────────────────────┘

The namespace creator UID (iow, the namespace owner) is irrelevant
here; the capability model is somewhat inconsistent here. Normal
capability checks that go down to cap_capable() (like ns_capable())
grant all privileges to processes in parent namespaces that have an
EUID that matches the owner UID of one of the intermediate namespaces,
including the target namespace. But capable_wrt_inode_uidgid() always
requires the caller to have the specified capability in its own
namespace because, when operating on an inode, the concept of an
implicit "target namespace" doesn't really exist. (For a properly
consistent model, you'd probably need to let the caller explicity
specify the target namespace, but then that would somewhat break the
transparency of namespaces.) cap_convert_nscap() starts by checking
for capable_wrt_inode_uidgid().

>        As with a binary that has VFS_CAP_REVISION_2 file capabilities,
>        a  binary  with  VFS_CAP_REVISION_3  file  capabilities confers
>        capabilities to a process during execve().  However,  capabili‐
>        ties  are conferred only if the binary is executed by a process
>        that resides in a user namespace whose UID 0 maps to  the  root
>        user  ID  that is saved in the extended attribute, or when exe‐
>        cuted by a process that resides in descendant of such a  names‐

Nit: "in a descendant"?

>>> Likewise,
>>> +.BR getxattr(2)
>>> +results will be converted and simplified to show a VFS_CAP_REVISION_2
>>> +extended attribute, if a VFS_CAP_REVISION_3 applies to the caller's
>>> +namespace, or to map the VFS_CAP_REVISION_3 root user ID into the
>>> +caller's namespace.
> I haven't captured that last paragraph in my text. I'm not sure I
> understand the idea being presented. Serge, could you elaborate?

Summary: When you read a capability attribute with getxattr(), the
kernel will rewrite the returned value such that it looks the way it
would have to look if the filesystem was mounted in your user
namespace; just like how, when the attribute is written, the caller
provides an attribute value written as if the filesystem was mounted
in the caller's user namespace.
Conceptually, this is mostly the same as the UID conversions applied
by chown() and stat().
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