From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B5CA9C32750 for ; Mon, 5 Aug 2019 00:08:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E5A32133F for ; Mon, 5 Aug 2019 00:08:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564963700; bh=ELtaRQYN03pGgzAn25S94BLsINs/8JhNgrvKCuHoZ9k=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=2YiOD3LIyBLsCEs8na6SOFGjf1NoVc5bLDiFiEWosfHn05t1mjoYpSM57lTaW69tm FQL9ma9D8kZdkshzIVVywFFwDat+tq6fm6X6PCqL81uN9OBOngg4emwplGdq2zg6j8 cRj61MbBKAGnOydMLusBOM1c/NxUcNwpCmi9Cg4w= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726620AbfHEAIT (ORCPT ); Sun, 4 Aug 2019 20:08:19 -0400 Received: from mail.kernel.org ([198.145.29.99]:53750 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726645AbfHEAIT (ORCPT ); Sun, 4 Aug 2019 20:08:19 -0400 Received: from mail-wm1-f50.google.com (mail-wm1-f50.google.com [209.85.128.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DB33421850 for ; Mon, 5 Aug 2019 00:08:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564963698; bh=ELtaRQYN03pGgzAn25S94BLsINs/8JhNgrvKCuHoZ9k=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=1lNGEUIl3Sy6L8XV2ZXT2mzFXajGxVmwBMjcAMezsTd2UyMLfWNx+jK937G6JUBmU hU4YUEonOaDsRFF+ZDS1nZJdVLkotgK0jFdSbvN3nMxJWk9oJQPtAiC58ylUnwno4+ jUKS+FaGkf50gOn46LS+3frGXovVVIaIYXkxuWyA= Received: by mail-wm1-f50.google.com with SMTP id h19so5465739wme.0 for ; Sun, 04 Aug 2019 17:08:17 -0700 (PDT) X-Gm-Message-State: APjAAAX7lCIpL/kI+iPJfyR3eOYSu6+VOuQNLb+PUEzc9QDzuWKe7RM9 LuMe5xUzWkTf2XC5HbqnKRZJhMYDredmt+rZBm/U8w== X-Google-Smtp-Source: APXvYqyTNqjoISdyFb7w3JO/Bw9LihRAwbtwXz7803DLe1S75xWznr8s17YVvDInum6BOcYrxn+3JJDHH4vEeLdon/w= X-Received: by 2002:a1c:9a53:: with SMTP id c80mr14236270wme.173.1564963696165; Sun, 04 Aug 2019 17:08:16 -0700 (PDT) MIME-Version: 1.0 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> <4A7A225A-6C23-4C0F-9A95-7C6C56B281ED@fb.com> <514D5453-0AEE-420F-AEB6-3F4F58C62E7E@fb.com> <1DE886F3-3982-45DE-B545-67AD6A4871AB@amacapital.net> <7F51F8B8-CF4C-4D82-AAE1-F0F28951DB7F@fb.com> <77354A95-4107-41A7-8936-D144F01C3CA4@fb.com> <369476A8-4CE1-43DA-9239-06437C0384C7@fb.com> <5A2FCD7E-7F54-41E5-BFAE-BB9494E74F2D@fb.com> In-Reply-To: From: Andy Lutomirski Date: Sun, 4 Aug 2019 17:08:04 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf To: Andy Lutomirski Cc: Song Liu , Kees Cook , Networking , bpf , Alexei Starovoitov , Daniel Borkmann , Kernel Team , Lorenz Bauer , Jann Horn , Greg KH , Linux API , LSM List Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On Sun, Aug 4, 2019 at 3:16 PM Andy Lutomirski wrote: > > On Fri, Aug 2, 2019 at 12:22 AM Song Liu wrote: > > > > Hi Andy, > > > >> I actually agree CAP_BPF_ADMIN makes sense. The hard part is to make > > >> existing tools (setcap, getcap, etc.) and libraries aware of the new CAP. > > > > > > It's been done before -- it's not that hard. IMO the main tricky bit > > > would be try be somewhat careful about defining exactly what > > > CAP_BPF_ADMIN does. > > > > Agreed. I think defining CAP_BPF_ADMIN could be a good topic for the > > Plumbers conference. > > > > OTOH, I don't think we have to wait for CAP_BPF_ADMIN to allow daemons > > like systemd to do sys_bpf() without root. > > I don't understand the use case here. Are you talking about systemd > --user? As far as I know, a user is expected to be able to fully > control their systemd --user process, so giving it unrestricted bpf > access is very close to giving it superuser access, and this doesn't > sound like a good idea. I think that, if systemd --user needs bpf(), > it either needs real unprivileged bpf() or it needs a privileged > helper (SUID or a daemon) to intermediate this access. > > > > > > > > >>> I don't see why you need to invent a whole new mechanism for this. > > >>> The entire cgroup ecosystem outside bpf() does just fine using the > > >>> write permission on files in cgroupfs to control access. Why can't > > >>> bpf() do the same thing? > > >> > > >> It is easier to use write permission for BPF_PROG_ATTACH. But it is > > >> not easy to do the same for other bpf commands: BPF_PROG_LOAD and > > >> BPF_MAP_*. A lot of these commands don't have target concept. Maybe > > >> we should have target concept for all these commands. But that is a > > >> much bigger project. OTOH, "all or nothing" model allows all these > > >> commands at once. > > > > > > For BPF_PROG_LOAD, I admit I've never understood why permission is > > > required at all. I think that CAP_SYS_ADMIN or similar should be > > > needed to get is_priv in the verifier, but I think that should mainly > > > be useful for tracing, and that requires lots of privilege anyway. > > > BPF_MAP_* is probably the trickiest part. One solution would be some > > > kind of bpffs, but I'm sure other solutions are possible. > > > > Improving permission management of cgroup_bpf is another good topic to > > discuss. However, it is also an overkill for current use case. > > > > I looked at the code some more, and I don't think this is so hard > after all. As I understand it, all of the map..by_id stuff is, to > some extent, deprecated in favor of persistent maps. As I see it, the > map..by_id calls should require privilege forever, although I can > imagine ways to scope that privilege to a namespace if the maps > themselves were to be scoped to a namespace. > > Instead, unprivileged tools would use the persistent map interface > roughly like this: > > $ bpftool map create /sys/fs/bpf/my_dir/filename type hash key 8 value > 8 entries 64 name mapname > > This would require that the caller have either CAP_DAC_OVERRIDE or > that the caller have permission to create files in /sys/fs/bpf/my_dir > (using the same rules as for any filesystem), and the resulting map > would end up owned by the creating user and have mode 0600 (or maybe > 0666, or maybe a new bpf_attr parameter) modified by umask. Then all > the various capable() checks that are currently involved in accessing > a persistent map would instead check FMODE_READ or FMODE_WRITE on the > map file as appropriate. > > Half of this stuff already works. I just set my system up like this: > > $ ls -l /sys/fs/bpf > total 0 > drwxr-xr-x. 3 luto luto 0 Aug 4 15:10 luto > > $ mkdir /sys/fs/bpf/luto/test > > $ ls -l /sys/fs/bpf/luto > total 0 > drwxrwxr-x. 2 luto luto 0 Aug 4 15:10 test > > I bet that making the bpf() syscalls work appropriately in this > context without privilege would only be a couple of hours of work. > The hard work, creating bpffs and making it function, is already done > :) > > P.S. The docs for bpftool create are less than fantastic. The > complete lack of any error message at all when the syscall returns > -EACCES is also not fantastic. This isn't remotely finished, but I spent a bit of time fiddling with this: https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=bpf/perms What do you think? (It's obviously not done. It doesn't compile, and I haven't gotten to the permissions needed to do map operations. I also haven't touched the capable() checks.)