On Tue, Oct 02, 2018 at 01:18:30PM +0200, Ondrej Mosnacek wrote:
No. With the side of Hell, No. The bug is real, but this is not the way to fix it.
First of all, it's still broken - e.g. mount something on a subdirectory and watch what that thing will do to it. And anyone who has permission to reload policy _will_ have one for mount.
And yes, debugfs_remove() suffers the same problem. Frankly, the only wish debugfs_remove() implementation inspires is to shoot it before it breeds. Which is the second problem with that approach - such stuff really shouldn't be duplicated in individual filesystems.
Don't copy (let along open-code) d_walk() in there. The guts of dcache tree handling are subtle enough (and had more than enough locking bugs over the years) to make spreading the dependencies on its details all over the tree an invitation for massive PITA down the road.
I have beginnings of patch doing that for debugfs; the same thing should be usable for selinuxfs as well. However, the main problem with selinuxfs wrt policy loading is different - what to do if sel_make_policy_nodes() fails halfway through? And what to do with accesses to the unholy mix of old and new nodes we currently have left behind?
Before security_load_policy() we don't know which nodes to create; after it we have nothing to fall back onto. It looks like we need to split security_load_policy() into "load", "switch over" and "free old" parts, so that the whole thing would look like load create new directory trees, unconnected to the root so they couldn't be reached by lookups until we are done switch over move the new directory trees in place kill the old trees (using that invalidate+genocide carefully combination) free old data structures with failures upon the first two steps handled by killing whatever detached trees we'd created (if any) and freeing the new data structures.
However, I'd really like to have the folks familiar with selinux guts to comment upon the feasibility of the above. AFAICS, nobody has ever seriously looked at that code wrt graceful error handling, etc.[*], so I'm not happy with making inferences from what the existing code is doing.
If you are interested in getting selinuxfs into sane shape, that would be a good place to start. As for the kernel-side rm -rf (which is what debugfs_remove() et.al. are trying to be)... * it absolutely needs to be in fs/*.c - either dcache or libfs. It's too closely tied to dcache guts to do otherwise. * as the first step it needs to do d_invalidate(), to get rid of anything that might be mounted on it and to prevent new mounts from appearing. It's rather tempting to unhash everything in the victim tree at the same time, but that needs to be done with care - I'm still not entirely happy with the solution I've got in that area. Alternative is to unhash them on the way out of subtree. simple_unlink()/simple_rmdir() are wrong there - we don't want to bother with the parent's timestamps as we go, for one thing; that should be done only once to parent of the root of that subtree. For another, we bloody well enforce the emptiness ourselves, so this simple_empty() is pointless (especially since we have no choice other than ignoring it anyway).
BTW, another selinuxfs unpleasantness is, the things like sel_write_enforce() don't have any exclusion against themselves, let alone the policy reloads. And quite a few of them obviously expect that e.g. permission check is done against the same policy the operation will apply to, not the previous one. That one definitely needs selinux folks involved.
[*] not too unreasonably so - anyone who gets to use _that_ as an attack vector has already won, so it's not a security problem pretty much by definition and running into heavy OOM at the time of policy reload is almost certainly going to end up with the userland parts of the entire thing not handling failures gracefully.