On Jul 15, 2015 3:34 PM, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> wrote: > > Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > > > On Wed, Jul 15, 2015 at 04:06:35PM -0500, Eric W. Biederman wrote: > >> Casey Schaufler <casey@xxxxxxxxxxxxxxxx> writes: > >> > >> > On 7/15/2015 12:46 PM, Seth Forshee wrote: > >> >> These are the first in a larger set of patches that I've been working on > >> >> (with help from Eric Biederman) to support mounting ext4 and fuse > >> >> filesystems from within user namespaces. I've pushed the full series to: > >> >> > >> >> git://kernel.ubuntu.com/sforshee/linux.git userns-mounts > >> >> > >> >> Taking the series as a whole, the strategy is to handle as much of the > >> >> heavy lifting as possible in the vfs so the filesystems don't have to > >> >> handle weird edge cases. If you look at the full series you'll find that > >> >> the changes in ext4 to support user namespace mounts turn out to be > >> >> fairly minimal (fuse is a bit more complicated though as it must deal > >> >> with translating ids for a userspace process which is running in pid and > >> >> user namespaces). > >> >> > >> >> The patches I'm sending today lay some of the groundwork in the vfs and > >> >> related code. They fall into two broad groups: > >> >> > >> >> 1. Patches 1-2 add s_user_ns and simplify MNT_NODEV handling. These are > >> >> pretty straightforward, and Eric has expressed interest in merging > >> >> these patches soon. Note that patch 2 won't apply cleanly without > >> >> Eric's noexec patches for proc and sys [1]. > >> >> > >> >> 2. Patches 2-7 tighten down security for mounts with s_user_ns != > >> >> &init_user_ns. This includes updates to how file caps and suid are > >> >> handled and LSM updates to ignore security labels on superblocks > >> >> from non-init namespaces. > >> >> > >> >> The LSM changes in particular may not be optimal, as I don't have a > >> >> lot of familiarity with this code, so I'd be especially appreciative > >> >> of review of these changes and suggestions on how to improve them. > >> > > >> > Lukasz Pawelczyk <l.pawelczyk@xxxxxxxxxxx> proposed > >> > LSM support in user namespaces ([RFC] lsm: namespace hooks) > >> > that make a whole lot more sense than just turning off > >> > the option of using labels on files. Gutting the ability > >> > to use MAC in a namespace is a step down the road of > >> > making MAC and namespaces incompatible. > >> > >> This is not "turning off the option to use labels on files". > >> > >> This is supporting mounting filesystems like ext4 by unprivileged users > >> and not trusting the labels they set in the same way as we trust labels > >> on filesystems mounted by privileged users. > >> > >> The first step needs to be not trusting those labels and treating such > >> filesystems as filesystems without label support. I hope that is Seth > >> has implemented. > >> > >> In the long run we can do more interesting things with such filesystems > >> once the appropriate LSM policy is in place. > > > > Yes, this exactly. Right now it looks to me like the only safe thing to > > do with mounts from unprivileged users is to ignore the security labels, > > so that's what I'm trying to do with these changes. If there's some > > better thing to do, or some better way to do it, I'm more than happy to > > receive that feedback. > > Ugh. > > This made me realize that we have an interesting problem here. An > unprivileged mount of tmpfs probably needs to have > s_user_ns == &init_user_ns. > > Otherwise we will break security labels on tmpfs for no good reason. > ramfs and sysfs also seem to have similar concerns. > > Because they have no backing store we can trust those filesystems with > security labels. Plus for at least sysfs there is the security label > bleed through issue, that we need to make certain works. > > Perhaps these filesystems with trusted backing store need to call > "sget_userns(..., &init_user_ns)". > > If we don't get this right we will have significant regressions with > respect to security labels, and that is not ok. That's only a problem if there's anyone who sets security labels on such a mount. You need global caps to do that (I hope), which requires someone outside the userns to help, which means there's a good chance that literally no one does this. --Andy -- 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
