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From:   Paul Moore <paul@paul-moore.com>
Date:   Thu, 25 Aug 2022 18:10:52 -0400
Message-ID: <CAHC9VhS4ROEY6uBwJPaTKX_bLiDRCyFJ9_+_08gFP0VWF_s-bQ@mail.gmail.com>
Subject: Re: [PATCH v5 0/4] Introduce security_create_user_ns()
To:     Song Liu <songliubraving@fb.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Frederick Lawler <fred@cloudflare.com>,
        KP Singh <kpsingh@kernel.org>,
        "revest@chromium.org" <revest@chromium.org>,
        "jackmanb@chromium.org" <jackmanb@chromium.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        James Morris <jmorris@namei.org>,
        "stephen.smalley.work@gmail.com" <stephen.smalley.work@gmail.com>,
        "eparis@parisplace.org" <eparis@parisplace.org>,
        Shuah Khan <shuah@kernel.org>,
        "brauner@kernel.org" <brauner@kernel.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        bpf <bpf@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
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On Thu, Aug 25, 2022 at 5:58 PM Song Liu <songliubraving@fb.com> wrote:
> > On Aug 25, 2022, at 12:19 PM, Paul Moore <paul@paul-moore.com> wrote:
> >
> > On Thu, Aug 25, 2022 at 2:15 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> Paul Moore <paul@paul-moore.com> writes:
> >>> On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@hallyn.com> 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@mail.gmail.com/
> >
> >> 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.
> Failing create_user_ns() doesn't make the system more reliable.
> Specifically, we call create_user_ns() via two paths: fork/clone and
> unshare. For both paths, we need the userspace to use user_namespace,
> and to honor failed create_user_ns().
>
> On the other hand, I would echo that killing the process is not
> practical in some use cases. Specifically, allowing the application to
> run in a less secure environment for a short period of time might be
> much better than killing it and taking down the whole service. Of
> course, there are other cases that security is more important, and
> taking down the whole service is the better choice.
>
> I guess the ultimate solution is a way to enforce using user_namespace
> in the kernel (if it ever makes sense...).

The LSM framework, and the BPF and SELinux LSM implementations in this
patchset, provide a mechanism to do just that: kernel enforced access
controls using flexible security policies which can be tailored by the
distro, solution provider, or end user to meet the specific needs of
their use case.

-- 
paul-moore.com