linux-kernel - Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

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Message-ID: <20200324145155.GB2685@chromium.org>
Date:   Tue, 24 Mar 2020 15:51:55 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Stephen Smalley <stephen.smalley.work@...il.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

On 24-Mär 10:51, Stephen Smalley wrote:
> On Tue, Mar 24, 2020 at 10:42 AM KP Singh <kpsingh@...omium.org> wrote:
> >
> > On 24-Mär 10:37, Stephen Smalley wrote:
> > > On Mon, Mar 23, 2020 at 9:52 PM KP Singh <kpsingh@...omium.org> wrote:
> > > >
> > > > On 23-Mär 18:13, Casey Schaufler wrote:
> > > > > Have you given up on the "BPF must be last" requirement?
> > > >
> > > > Yes, we dropped it for as the BPF programs require CAP_SYS_ADMIN
> > > > anwyays so the position ~shouldn't~ matter. (based on some of the
> > > > discussions we had on the BPF_MODIFY_RETURN patches).
> > > >
> > > > However, This can be added later (in a separate patch) if really
> > > > deemed necessary.
> > >
> > > It matters for SELinux, as I previously explained.  A process that has
> > > CAP_SYS_ADMIN is not assumed to be able to circumvent MAC policy.
> > > And executing prior to SELinux allows the bpf program to access and
> > > potentially leak to userspace information that wouldn't be visible to
> > > the
> > > process itself. However, I thought you were handling the order issue
> > > by putting it last in the list of lsms?
> >
> > We can still do that if it does not work for SELinux.
> >
> > Would it be okay to add bpf as LSM_ORDER_LAST?
> >
> > LSMs like Landlock can then add LSM_ORDER_UNPRIVILEGED to even end up
> > after bpf?
> 
> I guess the question is whether we need an explicit LSM_ORDER_LAST or
> can just handle it via the default
> values for the lsm= parameter, where you are already placing bpf last
> IIUC?  If someone can mess with the kernel boot
> parameters, they already have options to mess with SELinux, so it is no worse...

Yeah, we do add BPF as the last LSM in the default list. So, I will
avoid adding LSM_ORDER_LAST for now.

- KP