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.)
