Re: [PATCH] userns/capability: Add user namespace capability

[Serge E. Hallyn]

On Sun, Oct 18, 2015 at 10:13:54PM +0200, Tobias Markus wrote:
> On 17.10.2015 23:55, Serge E. Hallyn wrote:
> > On Sat, Oct 17, 2015 at 05:58:04PM +0200, Tobias Markus wrote:
> >> Add capability CAP_SYS_USER_NS.
> >> Tasks having CAP_SYS_USER_NS are allowed to create a new user namespace
> >> when calling clone or unshare with CLONE_NEWUSER.
> >>
> >> Rationale:
> >>
> >> Linux 3.8 saw the introduction of unpriviledged user namespaces,
> >> allowing unpriviledged users (without CAP_SYS_ADMIN) to be a "fake" root
> >> inside a separate user namespace. Before that, any namespace creation
> >> required CAP_SYS_ADMIN (or, in practice, the user had to be root).
> >> Unfortunately, there have been some security-relevant bugs in the
> >> meantime. Because of the fairly complex nature of user namespaces, it is
> >> reasonable to say that future vulnerabilties can not be excluded. Some
> >> distributions even wholly disable user namespaces because of this.
> > 
> > Fwiw I'm not in favor of this.  Debian has a patch (I believe the one
> > I originally wrote for Ubuntu but which Ubuntu dropped long ago) adding a
> > sysctl, off by default, for enabling user namespaces.
> 
> While it certainly works, enabling a feature like this at runtime
> doesn't seem like a long term solution.

We shouldn't need a long-term solution.  Your concern is bugs.  After
some time surely we'll feel that we have achieved a stable solution?

> The fact that Debian added this patch in the first place already
> demonstrates that there is demand for a way to limit unpriviledged user

No it does not.  As i said, I wrote that patch originally in the very
early days, when wanting it turned off was much more understandable.
I do not know whether Debian would have written its own patch if I
hadn't.  (They may have)

> namespace creation. Please, don't get me wrong: I would *really like* to
> see widespread adoption and continued development of user namespaces!
> But the status quo remains: Distributions outright disabling user
> namespaces (e.g. Arch Linux) won't make it easier.
> 
> > 
> > Posix capabilities are intended for privileged actions, not for
> > actions which explicitly should not require privilege, but which
> > we feel are in development.
> > 
> 
> Certainly, in an ideal world, user namespaces will never lead to any
> kernel-level exploits. But reality is different: There *have been*
> serious kernel vulnerabilities due to user namespaces, and there *will
> be* serious kernel vulnerabilities due to user namespaces.

As there will be due to sctp and futex.

> Now, those are the alternatives imho:
> 
> * Status quo: Some distributions will disable user namespaces by default
> in some way or another. User wishing to use user namespaces will have to
> use a custom kernel or enable a sysctl flag that was patched in by the
> downstream developers. On distributions that enable user namespaces by
> default, even users that don't wish to use them in the first places will
> be affected by vulnerabilities.
> 
> * Adding a capabilitiy: First of all, there would be no need for any
> downstream patches or custom kernels. Users that wish to use user
> namespaces would only have to enable the capability on the affected
> executables, if that hasn't been done by the package maintainers
> already. Users that might not even know of user namespaces have their peace.
> 
> > In general, the feeling is that putting a feature like this behind a
> > wall will only slow down the finding of any bugs, so I think the goal
> > itself is questionable.  But the chosen means for achieving your goal
> > are definately wrong.
> 
> I'm not talking about removing user namespaces altogether or making them
> impossible to use - as I said above, user wouldn't notice anything in
> the best case. Replacing setuid binaries with capabilitiy-based ones has
> been done for quite some time now and I don't think anyone complained.

That's the opposite - making something easier to use with less privilege,
as opposed to requiring more.

> I honestly don't see why adding a new capability would slow down finding
> bugs. Not every program magically profits from user namespaces. Why
> would, say, GCC, date or vim improve by using user namespaces? My point

<shrug> this is irrelevant, but I could certainly envision value in
something like gcc, which takes arbitrary input, running as kuid -1
(not uid -1).  Or especially ffmpeg.  chromium.

> is that use cases for user namespaces won't magically rain down from
> Heaven just because it possible to use them without priviledge. And it
> is hardly difficult to add the capabilitiy to those applications that
> use user namespaces, is it? setcap cap_sys_user_ns+ep $binary doesn't
> sound very complicated to me.

But it requires root privilege, for something designed to not need root
privilege.

> I would actually say not adding this capability would slow down finding
> bugs since users are less inclined to enable the feature if they can't
> limit its security impact.
> 
> Furthermore, saying "let's enable this complex security-relevant feature
> by default and make it impossible to limit it to certain files so users
> will find more bugs" is fundamentally wrong approach to security imho.
> First, you aren't likely to get more bug reports because distributions
> aren't that risky. Second, even if you get more bug reports, _the damage
> is already done_. Sysadmins won't be that happy and will very likely
> disable the very feature that caused the damage in the first place.

Posix capabilities are privileges.  Like the privileges to create
device nodes, or open another user's files regardless of permission
settings, or the ability to read syslog.
We require privilege for things which affect shared resouces or would
otherwise impact other users.  We do not require privilege because
something may have bugs.  The user namespace is designed precisely to
*not* require privilege.

If you feel the userns *design* is inherently buggy, then suggest
ripping it out.  If you feel the implementation is buggy, then compile
it out, or upstreaming the sysctl could makes sense.  We can have those
discussions.  But assigning a privilege to it doesn't make sense.

I'm not saying this as someone who championed and acked the userns
patchet, but as the capabilities maintainer.

-serge
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