On Sun, Mar 06, 2016 at 04:07:49PM -0600, Eric W. Biederman wrote: > Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > > > On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote: > >> Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > >> > >> > On Mon, Jan 04, 2016 at 12:03:50PM -0600, Seth Forshee wrote: > >> >> The mounter of a filesystem should be privileged towards the > >> >> inodes of that filesystem. Extend the checks in > >> >> inode_owner_or_capable() and capable_wrt_inode_uidgid() to > >> >> permit access by users priviliged in the user namespace of the > >> >> inode's superblock. > >> > > >> > Eric - I've discovered a problem related to this patch. The patches > >> > you've already applied to your testing branch make it so that s_user_ns > >> > can be an unprivileged user for proc and kernfs-based mounts. In some > >> > cases DAC is the only thing protecting files in these mounts (ignoring > >> > MAC), and with this patch an unprivileged user could bypass DAC. > >> > > >> > There's a simple solution - always set s_user_ns to &init_user_ns for > >> > those filesystems. I think this is the right thing to do, since the > >> > backing store behind these filesystems are really kernel objects. But > >> > this would break the assumption behind your patch "userns: Simpilify > >> > MNT_NODEV handling" and cause a regression in mounting behavior. > >> > > >> > I've come up with several possible solutions for this conflict. > >> > > >> > 1. Drop this patch and keep on setting s_user_ns to unprivilged users. > >> > This would be unfortunate because I think this patch does make sense > >> > for most filesystems. > >> > 2. Restrict this patch so that a user privileged towards s_user_ns is > >> > only privileged towards the super blocks inodes if s_user_ns has a > >> > mapping for both i_uid and i_gid. This is better than (1) but still > >> > not ideal in my mind. > >> > 3. Drop your patch and maintain the current MNT_NODEV behavior. > >> > 4. Add a new s_iflags flag to indicate a super block is from an > >> > unprivileged mount, and use this in your patch instead of s_user_ns. > >> > > >> > Any preference, or any other ideas? > >> > >> In general this is only an issue if uids and gids on the filesystem > >> do not map into the user namespace. > > > > Yes, both capable_wrt_inode_uidgid and inode_owner_or_capable will > > return true for a privileged user in the current namespace if the ids > > map into that namespace. > > > >> Therefore the general fix is to limit the logic of checking for > >> capabilities in s_user_ns if we are dealing with INVALID_UID and > >> INVALID_GID. For proc and kernfs that should never be the case > >> so the problem becomes a non-issue. > >> > >> Further I would look at limiting that relaxation to just > >> inode_change_ok. So that we can easily wrap that check per filesystem > >> and deny the relaxation for proc and kernfs. proc and kernfs already > >> have wrappers for .setattr so denying changes when !uid_vaid and > >> !gid_valid would be a trivial addition, and ensure calamity does > >> not ensure. > >> > >> Furthmore by limiting any additional to inode_change_ok we keep > >> the work of the additional tests off of the fast paths. > > > > So then the inode would need to be chowned before a privileged user in a > > non-init namespace would be capable towards it. That seems workable. It > > looks like INVALID_UID and INVALID_GID do map into init_user_ns (which > > seems a bit odd) so real root remains capable towards those indoes. > > > > That seems okay to me then. > > If I was not clear I was suggesting that we allow a sufficiently > privileged user in the filesysteme's s_user_ns to allow chowning files > with INVALID_UID and INVALID_GID. Right, I got that. > The global root user would always be able to do that because unless > capabilities are dropped it is sufficiently privileged in ever user > namespace. Sure. I was just commenting on one result - that ns-root has to chown the file before being privileged wrt that file but global root does not, on account of the fact that the invalid ids are mapped in init_user_ns. Seth -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
