Re: [PATCH v5 0/4] Introduce security_create_user_ns()

[Serge E. Hallyn]

On Mon, Aug 29, 2022 at 05:33:04PM +0200, Christian Brauner wrote:
> On Fri, Aug 26, 2022 at 04:00:39PM -0500, Serge Hallyn wrote:
> > On Fri, Aug 26, 2022 at 05:00:51PM +0000, Song Liu wrote:
> > > 
> > > 
> > > > On Aug 26, 2022, at 8:24 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> > > > 
> > > > On Thu, Aug 25, 2022 at 09:58:46PM +0000, Song Liu wrote:
> > > >> 
> > > >> 
> > > >>> On Aug 25, 2022, at 12:19 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> > > >>> 
> > > >>> On Thu, Aug 25, 2022 at 2:15 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> > > >>>> Paul Moore <paul@xxxxxxxxxxxxxx> writes:
> > > >>>>> On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> > > >>>>>> I am hoping we can come up with
> > > >>>>>> "something better" to address people's needs, make everyone happy, and
> > > >>>>>> bring forth world peace.  Which would stack just fine with what's here
> > > >>>>>> for defense in depth.
> > > >>>>>> 
> > > >>>>>> You may well not be interested in further work, and that's fine.  I need
> > > >>>>>> to set aside a few days to think on this.
> > > >>>>> 
> > > >>>>> I'm happy to continue the discussion as long as it's constructive; I
> > > >>>>> think we all are.  My gut feeling is that Frederick's approach falls
> > > >>>>> closest to the sweet spot of "workable without being overly offensive"
> > > >>>>> (*cough*), but if you've got an additional approach in mind, or an
> > > >>>>> alternative approach that solves the same use case problems, I think
> > > >>>>> we'd all love to hear about it.
> > > >>>> 
> > > >>>> I would love to actually hear the problems people are trying to solve so
> > > >>>> that we can have a sensible conversation about the trade offs.
> > > >>> 
> > > >>> Here are several taken from the previous threads, it's surely not a
> > > >>> complete list, but it should give you a good idea:
> > > >>> 
> > > >>> https://lore.kernel.org/linux-security-module/CAHC9VhQnPAsmjmKo-e84XDJ1wmaOFkTKPjjztsOa9Yrq+AeAQA@xxxxxxxxxxxxxx/
> > > >>> 
> > > >>>> As best I can tell without more information people want to use
> > > >>>> the creation of a user namespace as a signal that the code is
> > > >>>> attempting an exploit.
> > > >>> 
> > > >>> Some use cases are like that, there are several other use cases that
> > > >>> go beyond this; see all of our previous discussions on this
> > > >>> topic/patchset.  As has been mentioned before, there are use cases
> > > >>> that require improved observability, access control, or both.
> > > >>> 
> > > >>>> As such let me propose instead of returning an error code which will let
> > > >>>> the exploit continue, have the security hook return a bool.  With true
> > > >>>> meaning the code can continue and on false it will trigger using SIGSYS
> > > >>>> to terminate the program like seccomp does.
> > > >>> 
> > > >>> Having the kernel forcibly exit the process isn't something that most
> > > >>> LSMs would likely want.  I suppose we could modify the hook/caller so
> > > >>> that *if* an LSM wanted to return SIGSYS the system would kill the
> > > >>> process, but I would want that to be something in addition to
> > > >>> returning an error code like LSMs normally do (e.g. EACCES).
> > > >> 
> > > >> I am new to user_namespace and security work, so please pardon me if
> > > >> anything below is very wrong. 
> > > >> 
> > > >> IIUC, user_namespace is a tool that enables trusted userspace code to 
> > > >> control the behavior of untrusted (or less trusted) userspace code. 
> > > > 
> > > > No.  user namespaces are not a way for more trusted code to control the
> > > > behavior of less trusted code.
> > > 
> > > Hmm.. In this case, I think I really need to learn more. 
> > > 
> > > Thanks for pointing out my misunderstanding.
> > 
> > (I thought maybe Eric would chime in with a better explanation, but I'll
> > fill it in for now :)
> > 
> > One of the main goals of user namespaces is to allow unprivileged users
> > to do things like chroot and mount, which are very useful development
> > tools, without needing admin privileges.  So it's almost the opposite
> > of what you said: rather than to enable trusted userspace code to control
> > the behavior of less trusted code, it's to allow less privileged code to
> > do things which do not affect other users, without having to assume *more*
> > privilege.
> > 
> > To be precise, the goals were:
> > 
> > 1. uid mapping - allow two users to both "use uid 500" without conflicting
> > 2. provide (unprivileged) users privilege over their own resources
> > 3. absolutely no extra privilege over other resources
> > 4. be able to nest
> > 
> > While (3) was technically achieved, the problem we have is that
> > (2) provides unprivileged users the ability to exercise kernel code
> > which they previously could not.
> 
> The consequence of the refusal to give users any way to control whether
> or not user namespaces are available to unprivileged users is that a
> non-significant number of distros still carry the same patch for about
> 10 years now that adds an unprivileged_userns_clone sysctl to restrict
> them to privileged users. That includes current Debian and Archlinux btw.

Hi Christian,

I'm wondering about your placement of this argument in the thread, and whether
you interpreted what I said above as an argument against this patchset, or
whether you're just expanding on what I said.

> The LSM hook is a simple way to allow administrators to control this and

(I think the "control" here is suboptimal, but I've not seen - nor
conceived of - anything better as of yet)

> will allow user namespaces to be enabled in scenarios where they
> would otherwise not be accepted precisely because they are available to
> unprivileged users.
> 
> I fully understand the motivation and usefulness in unprivileged
> scenarios but it's an unfounded fear that giving users the ability to
> control user namespace creation via an LSM hook will cause proliferation
> of setuid binaries (Ignoring for a moment that any fully unprivileged
> container with useful idmappings has to rely on the new{g,u}idmap setuid
> binaries to setup useful mappings anyway.) or decrease system safety let
> alone cause regressions (Which I don't think is an applicable term here
> at all.). Distros that have unprivileged user namespaces turned on by
> default are extremely unlikely to switch to an LSM profile that turns
> them off and distros that already turn them off will continue to turn
> them off whether or not that LSM hook is available.
> 
> It's much more likely that workloads that want to minimize their attack
> surface while still getting the benefits of user namespaces for e.g.
> service isolation will feel comfortable enabling them for the first time
> since they can control them via an LSM profile.