Ondrej Mosnacek <omosnace@xxxxxxxxxx> writes: > On Mon, Mar 20, 2023 at 2:59 PM Dominick Grift > <dominick.grift@xxxxxxxxxxx> wrote: >> >> Stephen Smalley <stephen.smalley.work@xxxxxxxxx> writes: >> >> > On Mon, Mar 20, 2023 at 3:25 AM Dominick Grift >> > <dominick.grift@xxxxxxxxxxx> wrote: >> >> >> >> >> >> Hi, >> >> >> >> I was reading this pull request [1] and looked into how I might be able >> >> to implement this in policy but there seem to be some technical >> >> difficulties. >> >> >> >> * I already use getfscon to seperate the systemd user.slice because the >> >> system manager delegates the user.slice to the user manager. >> >> >> >> (genfscon "cgroup2" "/user.slice" cgroupfile_context) >> >> >> >> In the past the proved to be a racy where systemd attempts to >> >> write before the object has the context associated with the genfscon. >> > >> > I don't understand how this could be racy - genfscon-assigned contexts >> > should be assigned when the dentry is first instantiated via >> > inode_donit_with_dentry and therefore the inode shouldn't be >> > accessible to userspace prior to this initial assignment AFAIK. >> > Possibly I am missing something. >> >> I recall encountering this sporadically, but I admit that it has been a >> while since I supressed it in policy. I might try to reproduce. AFAIK my >> policy is the only policy that actually labels some trees on cgroup2 fs >> with private types currently. >> >> > >> >> I decided to dontaudit attempts to write to the mislabeled object and >> >> it *seems* as if systemd retries until it can write it i.e. when the >> >> object carries the expected label and so that seems to work eventually >> >> but it looks fragile. >> >> >> >> * The challenge with memory pressure implementation [2] is that these >> >> "memory.pressure" files end up in random locations under >> >> "/system.slice" for example: >> >> >> >> /sys/fs/cgroup/system.slice/systemd-journald.service/memory.pressure >> >> >> >> Where in the above systemd-journald.service might be >> >> templated (systemd-journald@FOO.service). Point is that the path is >> >> random. genfscon does not support regex and glob. I can't do for example: >> >> >> >> (genfscon "cgroup2" "/system.slice/.*/memory.pressure" >> >> cgroupfile_context) >> >> >> >> Fortunately cgroup2fs supports relabeling but if systemd has to >> >> manually relabel the cgroup files then I would imagine that this is >> >> racy as well, and that does not really solve the underlying issue. >> >> >> >> I am looking for ideas and suggestions >> > >> > Optimally one of two things would happen: >> > 1. The kernel would label the inode correctly when it is first created >> > (e.g. by augmenting genfscon to support more general matching), or >> > 2. The userspace component that creates these files would label them >> > correctly at creation (via setfscreatecon() prior to creation). >> >> Agree but 1. would require regex/glob support for genfscon and 2. these >> files aren't "created" by userspace AFAIK and so setfscreatecon or >> automatic object type transitions are probably not an option here. >> >> > >> > Pardon my ignorance but what creates these files initially? The kernel >> > in response to some event or systemd or some other userspace >> > component? >> >> Yes AFAIK it is the former (psuedo filesystem similar to procfs, debugfs >> in that sense). This is also why I don't think that the PR mentioned is >> tested because cgroup2 fs labeling is done with genfscon and not fsuse >> trans or fsuse xattr so even if the files would be created by >> userspace (which I think is not the case) the specified automatic object >> type transition rule wouldnt work. > > Actually, type transitions on cgroupfs should work - I added special > hooks for kernfs just for that some time ago - see kernel commits > d0c9c153b4bd6963c8fcccbc0caa12e8fa8d971d..e19dfdc83b60f196e0653d683499f7bc5548128f. Interesting. I will try this out. Would this not require at least a "fsuse trans" statement in policy? https://github.com/SELinuxProject/refpolicy/blob/master/policy/modules/kernel/filesystem.te#L89 Also I am not sure if that support would make much sense on a filesystem where files are created my the kernel in reaction to some event. > > Not sure what's behind the genfscon label assignment race, though. > >> >> I think eventually we currently probably have little choice but to make systemd >> reset the context of said cgroup file manually. Just wanted to see if >> there are alternatives. >> >> > >> >> [1] https://github.com/SELinuxProject/refpolicy/pull/607 >> >> [2] https://github.com/systemd/systemd/blob/main/docs/MEMORY_PRESSURE.md >> >> -- >> gpg --locate-keys dominick.grift@xxxxxxxxxxx >> Key fingerprint = FCD2 3660 5D6B 9D27 7FC6 E0FF DA7E 521F 10F6 4098 >> Dominick Grift >> -- gpg --locate-keys dominick.grift@xxxxxxxxxxx Key fingerprint = FCD2 3660 5D6B 9D27 7FC6 E0FF DA7E 521F 10F6 4098 Dominick Grift
