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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 11 Feb 2020 15:26:12 -0800
Message-ID: <CAADnVQLsiWgSBXbuxmpkC9TS8d1aQRw2zDHG8J6E=kfcRoXtKQ@mail.gmail.com>
Subject: Re: BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add
 mutable hooks list for the BPF LSM]
To:     Jann Horn <jannh@google.com>
Cc:     KP Singh <kpsingh@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Thomas Garnier <thgarnie@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Thomas Garnier <thgarnie@chromium.org>,
        Michael Halcrow <mhalcrow@google.com>,
        Paul Turner <pjt@google.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Matthew Garrett <mjg59@google.com>,
        Christian Brauner <christian@brauner.io>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kernel Team <kernel-team@fb.com>
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On Tue, Feb 11, 2020 at 1:38 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Feb 11, 2020 at 09:33:49PM +0100, Jann Horn wrote:
> > >
> > > Got it. Then let's whitelist them ?
> > > All error injection points are marked with ALLOW_ERROR_INJECTION().
> > > We can do something similar here, but let's do it via BTF and avoid
> > > abusing yet another elf section for this mark.
> > > I think BTF_TYPE_EMIT() should work. Just need to pick explicit enough
> > > name and extensive comment about what is going on.
> >
> > Sounds reasonable to me. :)
>
> awesome :)

Looks like the kernel already provides this whitelisting.
$ bpftool btf dump file /sys/kernel/btf/vmlinux |grep FUNC|grep '\<security_'
gives the list of all LSM hooks that lsm-bpf will be able to attach to.
There are two exceptions there security_add_hooks() and security_init().
Both are '__init'. Too late for lsm-bpf to touch.
So filtering BTF funcs by 'security_' prefix will be enough.
It should be documented though.
The number of attachable funcs depends on kconfig which is
a nice property and further strengthen the point that
lsm-bpf is very much kernel specific.
We probably should blacklist security_bpf*() hooks though.
Otherwise inception fans will have a field day.
Disallowing bpf with bpf :)