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

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Date:   Tue, 24 Mar 2020 10:57:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     KP Singh <kpsingh@...omium.org>
Cc:     Stephen Smalley <stephen.smalley.work@...il.com>,
        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>, 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 Tue, Mar 24, 2020 at 03:51:55PM +0100, KP Singh wrote:
> 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.

FWIW, this is my preference as well. If there ends up being a stronger
need, then we can implement LSM_ORDER_LAST at that time.

-- 
Kees Cook