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. Thanks, 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
