From: Andy Lutomirski <luto@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Song Liu <songliubraving@fb.com>,
Kees Cook <keescook@chromium.org>,
Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <Kernel-team@fb.com>,
Lorenz Bauer <lmb@cloudflare.com>, Jann Horn <jannh@google.com>,
Greg KH <gregkh@linuxfoundation.org>,
Linux API <linux-api@vger.kernel.org>,
LSM List <linux-security-module@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf
Date: Sun, 4 Aug 2019 22:47:22 -0700 [thread overview]
Message-ID: <CALCETrWtE2U4EvZVYeq8pSmQjBzF2PHH+KxYW8FSeF+W=1FYjw@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUjh6DdgW1qSuSRd1_=0F9CqB8+sNj__e_6AHEvh_BaxQ@mail.gmail.com>
On Sun, Aug 4, 2019 at 5:08 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Sun, Aug 4, 2019 at 3:16 PM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Fri, Aug 2, 2019 at 12:22 AM Song Liu <songliubraving@fb.com> 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.)
I updated the branch. It compiles, and basic map functionality works!
# mount -t bpf bpf /sys/fs/bpf
# cd /sys/fs/bpf
# mkdir luto
# chown luto: luto
# setpriv --euid=1000 --ruid=1000 bash
$ pwd
/sys/fs/bpf
bash-5.0$ ls -l
total 0
drwxr-xr-x 2 luto luto 0 Aug 4 22:41 luto
bash-5.0$ bpftool map create /sys/fs/bpf/luto/filename type hash key 8
value 8 entries 64 name mapname
bash-5.0$ bpftool map dump pinned /sys/fs/bpf/luto/filename
Found 0 elements
# chown root: /sys/fs/bpf/luto/filename
$ bpftool map dump pinned /sys/fs/bpf/luto/filename
Error: bpf obj get (/sys/fs/bpf/luto): Permission denied
So I think it's possible to get a respectable subset of bpf()
functionality working without privilege in short order :)
(FWIW, a decent fraction of this probably works even without my
patches, but it's going to have nonsensical semantics and may fail for
silly reasons.)
next prev parent reply other threads:[~2019-08-05 5:47 UTC|newest]
Thread overview: 92+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-27 20:19 [PATCH v2 bpf-next 0/4] sys_bpf() access control via /dev/bpf Song Liu
2019-06-27 20:19 ` [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access " Song Liu
2019-06-27 23:40 ` Andy Lutomirski
2019-06-27 23:42 ` Andy Lutomirski
2019-06-28 10:28 ` Christian Brauner
2019-06-28 9:05 ` Lorenz Bauer
2019-06-28 19:04 ` Song Liu
2019-06-30 0:12 ` Andy Lutomirski
2019-07-01 9:03 ` Song Liu
2019-07-02 1:59 ` Andy Lutomirski
2019-07-02 18:24 ` Kees Cook
2019-07-02 21:32 ` Andy Lutomirski
2019-07-02 23:48 ` Song Liu
2019-07-22 20:53 ` Song Liu
2019-07-23 10:45 ` Lorenz Bauer
2019-07-23 15:11 ` Andy Lutomirski
2019-07-23 22:56 ` Song Liu
2019-07-24 1:40 ` Andy Lutomirski
2019-07-24 6:30 ` Song Liu
2019-07-27 18:20 ` Song Liu
2019-07-30 5:07 ` Song Liu
2019-07-30 20:24 ` Andy Lutomirski
2019-07-31 8:10 ` Song Liu
2019-07-31 19:09 ` Andy Lutomirski
2019-08-02 7:21 ` Song Liu
2019-08-04 22:16 ` Andy Lutomirski
2019-08-05 0:08 ` Andy Lutomirski
2019-08-05 5:47 ` Andy Lutomirski [this message]
2019-08-05 7:36 ` Song Liu
2019-08-05 17:23 ` Andy Lutomirski
2019-08-05 19:21 ` Alexei Starovoitov
2019-08-05 21:25 ` Andy Lutomirski
2019-08-05 22:21 ` Andy Lutomirski
2019-08-06 1:11 ` Alexei Starovoitov
2019-08-07 5:24 ` Andy Lutomirski
2019-08-07 9:03 ` Lorenz Bauer
2019-08-07 13:52 ` Andy Lutomirski
2019-08-13 21:58 ` Alexei Starovoitov
2019-08-13 22:26 ` Daniel Colascione
2019-08-13 23:24 ` Andy Lutomirski
2019-08-13 23:06 ` Andy Lutomirski
2019-08-14 0:57 ` Alexei Starovoitov
2019-08-14 17:51 ` Andy Lutomirski
2019-08-14 22:05 ` Alexei Starovoitov
2019-08-14 22:30 ` Andy Lutomirski
2019-08-14 23:33 ` Alexei Starovoitov
2019-08-14 23:59 ` Andy Lutomirski
2019-08-15 0:36 ` Alexei Starovoitov
2019-08-15 11:24 ` Jordan Glover
2019-08-15 17:28 ` Alexei Starovoitov
2019-08-15 18:36 ` Andy Lutomirski
2019-08-15 23:08 ` Alexei Starovoitov
2019-08-16 9:34 ` Jordan Glover
2019-08-16 9:59 ` Thomas Gleixner
2019-08-16 11:33 ` Jordan Glover
2019-08-16 19:52 ` Alexei Starovoitov
2019-08-16 20:28 ` Thomas Gleixner
2019-08-17 15:02 ` Alexei Starovoitov
2019-08-17 15:44 ` Andy Lutomirski
2019-08-19 9:15 ` Thomas Gleixner
2019-08-19 17:27 ` Alexei Starovoitov
2019-08-19 17:38 ` Andy Lutomirski
2019-08-15 18:43 ` Jordan Glover
2019-08-15 19:46 ` Kees Cook
2019-08-15 23:46 ` Alexei Starovoitov
2019-08-16 0:54 ` Andy Lutomirski
2019-08-16 5:56 ` Song Liu
2019-08-16 21:45 ` Alexei Starovoitov
2019-08-16 22:22 ` Christian Brauner
2019-08-17 15:08 ` Alexei Starovoitov
2019-08-17 15:16 ` Christian Brauner
2019-08-17 15:36 ` Alexei Starovoitov
2019-08-17 15:42 ` Christian Brauner
2019-08-22 14:17 ` Daniel Borkmann
2019-08-22 15:16 ` Andy Lutomirski
2019-08-22 15:17 ` RFC: very rough draft of a bpf permission model Andy Lutomirski
2019-08-22 23:26 ` Alexei Starovoitov
2019-08-23 23:09 ` Andy Lutomirski
2019-08-26 22:36 ` Alexei Starovoitov
2019-08-27 0:05 ` Andy Lutomirski
2019-08-27 0:34 ` Alexei Starovoitov
2019-08-22 22:48 ` [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf Alexei Starovoitov
2019-07-30 20:20 ` Andy Lutomirski
2019-07-31 7:44 ` Song Liu
2019-06-28 9:01 ` Lorenz Bauer
2019-06-28 19:10 ` Song Liu
2019-07-01 9:34 ` Lorenz Bauer
2019-07-02 19:22 ` Andrii Nakryiko
2019-07-03 7:28 ` Greg KH
2019-06-27 20:19 ` [PATCH v2 bpf-next 2/4] bpf: sync tools/include/uapi/linux/bpf.h Song Liu
2019-06-27 20:19 ` [PATCH v2 bpf-next 3/4] libbpf: add libbpf_[enable|disable]_sys_bpf() Song Liu
2019-06-27 20:19 ` [PATCH v2 bpf-next 4/4] bpftool: use libbpf_[enable|disable]_sys_bpf() Song Liu
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