ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Stéphane Graber]

On Tue, Apr 29, 2014 at 04:22:55PM -0700, Andy Lutomirski wrote:
> On Tue, Apr 29, 2014 at 4:20 PM, Marian Marinov <mm at 1h.com> wrote:
> > On 04/30/2014 01:45 AM, Andy Lutomirski wrote:
> >>
> >> On 04/29/2014 03:29 PM, Serge Hallyn wrote:
> >>>
> >>> Quoting Marian Marinov (mm-108MBtLGafw at public.gmane.org):
> >>>>
> >>>> On 04/30/2014 01:02 AM, Serge Hallyn wrote:
> >>>>>
> >>>>> Quoting Marian Marinov (mm-108MBtLGafw at public.gmane.org):
> >>>>>>
> >>>>>> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
> >>>>>>>
> >>>>>>> Quoting Theodore Ts'o (tytso-3s7WtUTddSA at public.gmane.org):
> >>>>>>>>
> >>>>>>>> On Tue, Apr 29, 2014 at 04:49:14PM +0300, Marian Marinov wrote:
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> I'm proposing a fix to this, by replacing the
> >>>>>>>>> capable(CAP_LINUX_IMMUTABLE)
> >>>>>>>>> check with ns_capable(current_cred()->user_ns,
> >>>>>>>>> CAP_LINUX_IMMUTABLE).
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> Um, wouldn't it be better to simply fix the capable() function?
> >>>>>>>>
> >>>>>>>> /**
> >>>>>>>>   * capable - Determine if the current task has a superior
> >>>>>>>> capability in effect
> >>>>>>>>   * @cap: The capability to be tested for
> >>>>>>>>   *
> >>>>>>>>   * Return true if the current task has the given superior
> >>>>>>>> capability currently
> >>>>>>>>   * available for use, false if not.
> >>>>>>>>   *
> >>>>>>>>   * This sets PF_SUPERPRIV on the task if the capability is
> >>>>>>>> available on the
> >>>>>>>>   * assumption that it's about to be used.
> >>>>>>>>   */
> >>>>>>>> bool capable(int cap)
> >>>>>>>> {
> >>>>>>>>         return ns_capable(&init_user_ns, cap);
> >>>>>>>> }
> >>>>>>>> EXPORT_SYMBOL(capable);
> >>>>>>>>
> >>>>>>>> The documentation states that it is for "the current task", and I
> >>>>>>>> can't imagine any use case, where user namespaces are in effect,
> >>>>>>>> where
> >>>>>>>> using init_user_ns would ever make sense.
> >>>>>>>
> >>>>>>>
> >>>>>>> the init_user_ns represents the user_ns owning the object, not the
> >>>>>>> subject.
> >>>>>>>
> >>>>>>> The patch by Marian is wrong.  Anyone can do 'clone(CLONE_NEWUSER)',
> >>>>>>> setuid(0), execve, and end up satisfying
> >>>>>>> 'ns_capable(current_cred()->userns,
> >>>>>>> CAP_SYS_IMMUTABLE)' by definition.
> >>>>>>>
> >>>>>>> So NACK to that particular patch.  I'm not sure, but IIUC it should
> >>>>>>> be
> >>>>>>> safe to check against the userns owning the inode?
> >>>>>>>
> >>>>>>
> >>>>>> So what you are proposing is to replace
> >>>>>> 'ns_capable(current_cred()->userns, CAP_SYS_IMMUTABLE)' with
> >>>>>> 'inode_capable(inode, CAP_SYS_IMMUTABLE)' ?
> >>>>>>
> >>>>>> I agree that this is more sane.
> >>>>>
> >>>>>
> >>>>> Right, and I think the two operations you're looking at seem sane
> >>>>> to allow.
> >>>>
> >>>>
> >>>> If you are ok with this patch, I will fix all file systems and send
> >>>> patches.
> >>>
> >>>
> >>> Sounds good, thanks.
> >>>
> >>>> Signed-off-by: Marian Marinov <mm-NV7Lj0SOnH0 at public.gmane.org>
> >>>
> >>>
> >>> Acked-by: Serge E. Hallyn
> >>> <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA at public.gmane.org>
> >>
> >>
> >> Wait, what?
> >>
> >> Inodes aren't owned by user namespaces; they're owned by users.  And any
> >> user can arrange to have a user namespace in which they pass an
> >> inode_capable check on any inode that they own.
> >>
> >> Presumably there's a reason that CAP_SYS_IMMUTABLE is needed.  If this
> >> gets merged, then it would be better to just drop CAP_SYS_IMMUTABLE
> >> entirely.
> >
> >
> > The problem I'm trying to solve is this:
> >
> > container with its own user namespace and CAP_SYS_IMMUTABLE should be able
> > to use chattr on all files witch this container has access to.
> >
> > Unfortunately with the capable(CAP_SYS_IMMUTABLE) check this is not working.
> >
> > With the proposed two fixes CAP_SYS_IMMUTABLE started working in the
> > container.
> >
> > The first solution got its user namespace from the currently running process
> > and the second gets its user namespace from the currently opened inode.
> >
> > So what would be the best solution in this case?
> 
> I'd suggest adding a mount option like fs_owner_uid that names a uid
> that owns, in the sense of having unlimited access to, a filesystem.
> Then anyone with caps on a namespace owned by that uid could do
> whatever.
> 
> Eric?
> 
> --Andy

The most obvious problem I can think of with "do whatever" is that this
will likely include mknod of char and block devices which you can then
chown/chmod as you wish and use to access any devices on the system from
an unprivileged container.
This can however be mitigated by using the devices cgroup controller.

You also probably wouldn't want any unprivileged user from the host to
find a way to access that mounted filesytem but so long as you do the
mount in a separate mountns and don't share uids between the host and
the container, that should be fine too.

-- 
Stéphane Graber
Ubuntu developer
http://www.ubuntu.com
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