Hi Serge, "Serge E. Hallyn" <serge@xxxxxxxxxx> writes: > On Thu, Apr 22, 2021 at 11:18:01AM +0200, Giuseppe Scrivano wrote: >> Snaipe <snaipe@xxxxxxxxxx> writes: >> >> > "Giuseppe Scrivano" <gscrivan@xxxxxxxxxx> writes: >> >>>> >> instead of a prctl, I've added a new mode to /proc/PID/setgroups that >> >>>> >> allows setgroups in a userns locking the current gids. >> >>>> >> >> >>>> >> What do you think about using /proc/PID/setgroups instead of a new >> >>>> >> prctl()? >> >>>> > >> >>>> > It's better than not having it, but two concerns - >> >>>> > >> >>>> > 1. some userspace, especially testsuites, could become confused by the fact >> >>>> > that they can't drop groups no matter how hard they try, since these will all >> >>>> > still show up as regular groups. >> >>>> >> >>>> I forgot to send a link to a second patch :-) that completes the feature: >> >>>> https://github.com/giuseppe/linux/commit/1c5fe726346b216293a527719e64f34e6297f0c2 >> >>>> >> >>>> When the new mode is used, the gids that are not known in the userns do >> >>>> not show up in userspace. >> >>> >> >>> Ah, right - and of course those gids better not be mapped into the namespace :) >> >>> >> >>> But so, this is the patch you said you agreed was not worth the extra >> >>> complexity? >> >> >> >> yes, these two patches are what looked too complex at that time. The >> >> problem still exists though, we could perhaps reconsider if the >> >> extra-complexity is acceptable to address it. >> > >> > Hey Folks, sorry for necro-bumping, but I've found this discussion >> > while searching for this specific issue, and it seems like the most >> > recent relevant discussion on the matter. I'd like to chime in with >> > our personal experience. >> > >> > We have a tool[1] that allows unprivileged use of namespaces >> > (when using a userns, which is the default). >> > >> > The primary use-case of said tool is lightweight containerization, >> > but we're also using it for other mundane usages, like a better >> > substitute for fakeroot to build and package privileged software >> > (e.g. sudo or ping, which needs to be installed with special >> > capabilities) unprivileged, or to copy file trees that are owned by >> > the user or sub-ids. >> > >> > For the first use-case, it's always safe to drop unmapped groups, >> > because the target rootfs is always owned by the user or its sub-ids. >> > >> > For the other use-cases, this is more problematic, as you're all >> > well-aware of. Our position right now is that the tool will always >> > allow setgroups in user namespace, and that it's not safe to use on >> > systems that rely on negative access groups. >> > >> > I think that something that's not mentioned is that if a user setgroups >> > to a fixed list of subgids, dropping all unmapped gids, they don't just >> > gain the ability to access these negative-access files, they also lose >> > legitimate access to files that their unmapped groups allow them to >> > access. This is fine for our first use-case, but a bit surprising for >> > the second one -- and since setgroups never lets us keep unmapped gids, >> > we have no way to keep these desired groups. >> > >> > From a first glance, a sysctl that explicitly controls that would not >> > address the above problem, but keeping around the original group list >> > of the owner of the user ns would have the desired semantics. >> > >> > Giuseppe's patch seems to address this use case, which would personally >> > make me very happy. >> > >> > [1]: https://github.com/aristanetworks/bst >> >> thanks for the feedback. We are still facing the issue with rootless >> Podman, and these patches (listed here so you won't need to dig into archives): >> >> https://github.com/giuseppe/linux/commit/7e0701b389c497472d11fab8570c153a414050af >> https://github.com/giuseppe/linux/commit/1c5fe726346b216293a527719e64f34e6297f0c2 >> >> would solve the issue for us as well and we can use setgroups within a >> user namespace in a safe way. >> >> Any comments on this approach? Could we move forward with it? > > Can you send these to lkml? I'll have to go back through our previous > discussions, but offhand this looks good to me. I've just sent them to lkml. Regards, Giuseppe
