linux-kernel - Re: [PATCH v5 0/4] Introduce security_create_user

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Date:   Thu, 25 Aug 2022 15:19:09 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederick Lawler <fred@...udflare.com>, kpsingh@...nel.org,
        revest@...omium.org, jackmanb@...omium.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        jmorris@...ei.org, stephen.smalley.work@...il.com,
        eparis@...isplace.org, shuah@...nel.org, brauner@...nel.org,
        casey@...aufler-ca.com, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel-team@...udflare.com,
        cgzones@...glemail.com, karl@...badwolfsecurity.com,
        tixxdz@...il.com
Subject: Re: [PATCH v5 0/4] Introduce security_create_user_ns()

On Thu, Aug 25, 2022 at 2:15 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Paul Moore <paul@...l-moore.com> writes:
> > On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@...lyn.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).

-- 
paul-moore.com