Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

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On Sun, Aug 4, 2019 at 3:16 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Fri, Aug 2, 2019 at 12:22 AM Song Liu <songliubraving@xxxxxx> 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.)



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