On Wed, Apr 8, 2020 at 2:21 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: > > > 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. > > Why, exactly? (also, see below) For the reasons I explained below: because you can turn read-only bpf_link into writable one through pinning + chmod, if you have CAP_DAC_OVERRIDE. > > > 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. > > Ah, I did not know that was possible - I was assuming that bpffs was > doing something special to prevent that. But if not, great! > > > 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. > > Hmm, I disagree that deliberately making users jump through hoops is a > good thing. Smells an awful lot like security through obscurity to me; > and we all know how well that works anyway... Depends on who users are? bpftool can implement this as one of `bpftool link` sub-commands and allow human operators to force-detach bpf_link, if necessary. I think applications shouldn't do this (programmatically) at all, which is why I think it's actually good that it's harder and not obvious, this will make developer think again before implementing this, hopefully. For me it's about discouraging bad practice. > > >> 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?... > > Ah yes, so it is! I guess I skipped over that a bit too fast ;) > > > There are few bugs in the implementation currently, but it will work > > in the final version. > > Cool. > > >> > 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. > > Yeah, I do hope that we'll eventually get CAP_BPF... > > -Toke >
