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. The global root user would always be able to do that because unless capabilities are dropped it is sufficiently privileged in ever user namespace. Eric -- 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
