From mboxrd@z Thu Jan 1 00:00:00 1970 From: Song Liu Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf Date: Mon, 22 Jul 2019 20:53:48 +0000 Message-ID: <4A7A225A-6C23-4C0F-9A95-7C6C56B281ED@fb.com> References: <20190627201923.2589391-1-songliubraving@fb.com> <20190627201923.2589391-2-songliubraving@fb.com> <21894f45-70d8-dfca-8c02-044f776c5e05@kernel.org> <3C595328-3ABE-4421-9772-8D41094A4F57@fb.com> <0DE7F23E-9CD2-4F03-82B5-835506B59056@fb.com> <201907021115.DCD56BBABB@keescook> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: Content-Language: en-US Content-ID: <4934353FD63ADE4698F42238323400FC@namprd15.prod.outlook.com> Sender: netdev-owner@vger.kernel.org To: Andy Lutomirski Cc: Kees Cook , "linux-security@vger.kernel.org" , Networking , bpf , Alexei Starovoitov , Daniel Borkmann , Kernel Team , Lorenz Bauer , Jann Horn , Greg KH , Linux API List-Id: linux-api@vger.kernel.org Hi Andy, Lorenz, and all,=20 > On Jul 2, 2019, at 2:32 PM, Andy Lutomirski wrote: >=20 > On Tue, Jul 2, 2019 at 2:04 PM Kees Cook wrote: >>=20 >> On Mon, Jul 01, 2019 at 06:59:13PM -0700, Andy Lutomirski wrote: >>> I think I'm understanding your motivation. You're not trying to make >>> bpf() generically usable without privilege -- you're trying to create >>> a way to allow certain users to access dangerous bpf functionality >>> within some limits. >>>=20 >>> That's a perfectly fine goal, but I think you're reinventing the >>> wheel, and the wheel you're reinventing is quite complicated and >>> already exists. I think you should teach bpftool to be secure when >>> installed setuid root or with fscaps enabled and put your policy in >>> bpftool. If you want to harden this a little bit, it would seem >>> entirely reasonable to add a new CAP_BPF_ADMIN and change some, but >>> not all, of the capable() checks to check CAP_BPF_ADMIN instead of the >>> capabilities that they currently check. >>=20 >> If finer grained controls are wanted, it does seem like the /dev/bpf >> path makes the most sense. open, request abilities, use fd. The open can >> be mediated by DAC and LSM. The request can be mediated by LSM. This >> provides a way to add policy at the LSM level and at the tool level. >> (i.e. For tool-level controls: leave LSM wide open, make /dev/bpf owned >> by "bpfadmin" and bpftool becomes setuid "bpfadmin". For fine-grained >> controls, leave /dev/bpf wide open and add policy to SELinux, etc.) >>=20 >> With only a new CAP, you don't get the fine-grained controls. (The >> "request abilities" part is the key there.) >=20 > Sure you do: the effective set. It has somewhat bizarre defaults, but > I don't think that's a real problem. Also, this wouldn't be like > CAP_DAC_READ_SEARCH -- you can't accidentally use your BPF caps. >=20 > I think that a /dev capability-like object isn't totally nuts, but I > think we should do it well, and this patch doesn't really achieve > that. But I don't think bpf wants fine-grained controls like this at > all -- as I pointed upthread, a fine-grained solution really wants > different treatment for the different capable() checks, and a bunch of > them won't resemble capabilities or /dev/bpf at all. With 5.3-rc1 out, I am back on this. :) How about we modify the set as: 1. Introduce sys_bpf_with_cap() that takes fd of /dev/bpf.=20 2. Better handling of capable() calls through bpf code. I guess the biggest problem here is is_priv in verifier.c:bpf_check().=20 With this approach, we will be able to pass the fd around, so it should=20 also solve problem for Go.=20 Please let me know your comments/suggestions on this direction.=20 Thanks, Song