On 07/15/2015 09:05 PM, Andy Lutomirski wrote: > 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. Setting of security.selinux attributes is governed by SELinux permission checks, not by capabilities. Also, files are always assigned a label at creation time; a tmpfs inode will be labeled based on its creator without any userspace entity ever calling setxattr() at all. -- 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
