On Wed, Apr 8, 2020 at 8:14 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: > > > On Mon, Apr 6, 2020 at 4:34 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Andrii Nakryiko <andriin@xxxxxx> writes: > >> > >> > Add support to look up bpf_link by ID and iterate over all existing bpf_links > >> > in the system. GET_FD_BY_ID code handles not-yet-ready bpf_link by checking > >> > that its ID hasn't been set to non-zero value yet. Setting bpf_link's ID is > >> > done as the very last step in finalizing bpf_link, together with installing > >> > FD. This approach allows users of bpf_link in kernel code to not worry about > >> > races between user-space and kernel code that hasn't finished attaching and > >> > initializing bpf_link. > >> > > >> > Further, it's critical that BPF_LINK_GET_FD_BY_ID only ever allows to create > >> > bpf_link FD that's O_RDONLY. This is to protect processes owning bpf_link and > >> > thus allowed to perform modifications on them (like LINK_UPDATE), from other > >> > processes that got bpf_link ID from GET_NEXT_ID API. In the latter case, only > >> > querying bpf_link information (implemented later in the series) will be > >> > allowed. > >> > >> I must admit I remain sceptical about this model of restricting access > >> without any of the regular override mechanisms (for instance, enforcing > >> read-only mode regardless of CAP_DAC_OVERRIDE in this series). Since you > >> keep saying there would be 'some' override mechanism, I think it would > >> be helpful if you could just include that so we can see the full > >> mechanism in context. > > > > I wasn't aware of CAP_DAC_OVERRIDE, thanks for bringing this up. > > > > One way to go about this is to allow creating writable bpf_link for > > GET_FD_BY_ID if CAP_DAC_OVERRIDE is set. Then we can allow LINK_DETACH > > operation on writable links, same as we do with LINK_UPDATE here. > > LINK_DETACH will do the same as cgroup bpf_link auto-detachment on > > cgroup dying: it will detach bpf_link, but will leave it alive until > > last FD is closed. > > Yup, I think this would be a reasonable way to implement the override > mechanism - it would ensure 'full root' users (like a root shell) can > remove attachments, while still preventing applications from doing so by > limiting their capabilities. So I did some experiments and I think I want to keep GET_FD_BY_ID for bpf_link to return only read-only bpf_links. After that, one can pin bpf_link temporarily and re-open it as writable one, provided CAP_DAC_OVERRIDE capability is present. All that works already, because pinned bpf_link is just a file, so one can do fchmod on it and all that will go through normal file access permission check code path. Unfortunately, just re-opening same FD as writable (which would be possible if fcntl(fd, F_SETFL, S_IRUSR S_IWUSR) was supported on Linux) without pinning is not possible. Opening link from /proc/<pid>/fd/<link-fd> doesn't seem to work either, because backing inode is not BPF FS inode. I'm not sure, but maybe we can support the latter eventually. But either way, I think given this is to be used for manual troubleshooting, going through few extra hoops to force-detach bpf_link is actually a good thing. > > Extending on the concept of RO/RW bpf_link attachments, maybe it should > even be possible for an application to choose which mode it wants to pin > its fd in? With the same capability being able to override it of > course... Isn't that what patch #2 is doing?... There are few bugs in the implementation currently, but it will work in the final version. > > > We need to consider, though, if CAP_DAC_OVERRIDE is something that can > > be disabled for majority of real-life applications to prevent them > > from doing this. If every realistic application has/needs > > CAP_DAC_OVERRIDE, then that's essentially just saying that anyone can > > get writable bpf_link and do anything with it. > > I poked around a bit, and looking at the sandboxing configurations > shipped with various daemons in their systemd unit files, it appears > that the main case where daemons are granted CAP_DAC_OVERRIDE is if they > have to be able to read /etc/shadow (which is installed as chmod 0). If > this is really the case, that would indicate it's not a widely needed > capability; but I wouldn't exactly say that I've done a comprehensive > survey, so probably a good idea for you to check your users as well :) Right, it might not be possible to drop it for all applications right away, but at least CAP_DAC_OVERRIDE is not CAP_SYS_ADMIN, which is absolutely necessary to work with BPF. > > -Toke >
