Re: [PATCH 6/7] selinux: Ignore security labels on user namespace mounts

Seth Forshee
2015-07-30T16:24:38Z

On Thu, Jul 30, 2015 at 11:57:24AM -0400, Stephen Smalley wrote:
> On 07/24/2015 11:11 AM, Seth Forshee wrote:
> > On Thu, Jul 23, 2015 at 11:23:31AM -0500, Seth Forshee wrote:
> >> On Thu, Jul 23, 2015 at 11:36:03AM -0400, Stephen Smalley wrote:
> >>> On 07/23/2015 10:39 AM, Seth Forshee wrote:
> >>>> On Thu, Jul 23, 2015 at 09:57:20AM -0400, Stephen Smalley wrote:
> >>>>> On 07/22/2015 04:40 PM, Stephen Smalley wrote:
> >>>>>> On 07/22/2015 04:25 PM, Stephen Smalley wrote:
> >>>>>>> On 07/22/2015 12:14 PM, Seth Forshee wrote:
> >>>>>>>> On Wed, Jul 22, 2015 at 12:02:13PM -0400, Stephen Smalley wrote:
> >>>>>>>>> On 07/16/2015 09:23 AM, Stephen Smalley wrote:
> >>>>>>>>>> On 07/15/2015 03:46 PM, Seth Forshee wrote:
> >>>>>>>>>>> Unprivileged users should not be able to supply security labels
> >>>>>>>>>>> in filesystems, nor should they be able to supply security
> >>>>>>>>>>> contexts in unprivileged mounts. For any mount where s_user_ns is
> >>>>>>>>>>> not init_user_ns, force the use of SECURITY_FS_USE_NONE behavior
> >>>>>>>>>>> and return EPERM if any contexts are supplied in the mount
> >>>>>>>>>>> options.
> >>>>>>>>>>>
> >>>>>>>>>>> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> >>>>>>>>>>
> >>>>>>>>>> I think this is obsoleted by the subsequent discussion, but just for the
> >>>>>>>>>> record: this patch would cause the files in the userns mount to be left
> >>>>>>>>>> with the "unlabeled" label, and therefore under typical policies,
> >>>>>>>>>> completely inaccessible to any process in a confined domain.
> >>>>>>>>>
> >>>>>>>>> The right way to handle this for SELinux would be to automatically use
> >>>>>>>>> mountpoint labeling (SECURITY_FS_USE_MNTPOINT, normally set by
> >>>>>>>>> specifying a context= mount option), with the sbsec->mntpoint_sid set
> >>>>>>>>> from some related object (e.g. the block device file context, as in your
> >>>>>>>>> patches for Smack). That will cause SELinux to use that value instead
> >>>>>>>>> of any xattr value from the filesystem and will cause attempts by
> >>>>>>>>> userspace to set the security.selinux xattr to fail on that filesystem.
> >>>>>>>>> That is how SELinux normally deals with untrusted filesystems, except
> >>>>>>>>> that it is normally specified as a mount option by a trusted mounting
> >>>>>>>>> process, whereas in your case you need to automatically set it.
> >>>>>>>>
> >>>>>>>> Excellent, thank you for the advice. I'll start on this when I've
> >>>>>>>> finished with Smack.
> >>>>>>>
> >>>>>>> Not tested, but something like this should work. Note that it should
> >>>>>>> come after the call to security_fs_use() so we know whether SELinux
> >>>>>>> would even try to use xattrs supplied by the filesystem in the first place.
> >>>>>>>
> >>>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> >>>>>>> index 564079c..84da3a2 100644
> >>>>>>> --- a/security/selinux/hooks.c
> >>>>>>> +++ b/security/selinux/hooks.c
> >>>>>>> @@ -745,6 +745,30 @@ static int selinux_set_mnt_opts(struct super_block *sb,
> >>>>>>> goto out;
> >>>>>>> }
> >>>>>>> }
> >>>>>>> +
> >>>>>>> + /*
> >>>>>>> + * If this is a user namespace mount, no contexts are allowed
> >>>>>>> + * on the command line and security labels must be ignored.
> >>>>>>> + */
> >>>>>>> + if (sb->s_user_ns != &init_user_ns) {
> >>>>>>> + if (context_sid || fscontext_sid || rootcontext_sid ||
> >>>>>>> + defcontext_sid) {
> >>>>>>> + rc = -EACCES;
> >>>>>>> + goto out;
> >>>>>>> + }
> >>>>>>> + if (sbsec->behavior == SECURITY_FS_USE_XATTR) {
> >>>>>>> + struct block_device *bdev = sb->s_bdev;
> >>>>>>> + sbsec->behavior = SECURITY_FS_USE_MNTPOINT;
> >>>>>>> + if (bdev) {
> >>>>>>> + struct inode_security_struct *isec =
> >>>>>>> bdev->bd_inode;
> >>>>>>
> >>>>>> That should be bdev->bd_inode->i_security.
> >>>>>
> >>>>> Sorry, this won't work. bd_inode is not the inode of the block device
> >>>>> file that was passed to mount, and it isn't labeled in any way. It will
> >>>>> just be unlabeled.
> >>>>>
> >>>>> So I guess the only real option here as a fallback is
> >>>>> sbsec->mntpoint_sid = current_sid(). Which isn't great either, as the
> >>>>> only case where we currently assign task labels to files is for their
> >>>>> /proc/pid inodes, and no current policy will therefore allow create
> >>>>> permission to such files.
> >>>>
> >>>> Darn, you're right, that isn't the inode we want. There really doesn't
> >>>> seem to be any way to get back to the one we want from the LSM, short of
> >>>> adding a new hook.
> >>>
> >>> Maybe list_first_entry(&sb->s_bdev->bd_inodes, struct inode, i_devices)?
> >>> Feels like a layering violation though...
> >>
> >> Yeah, and even though that probably works out to be the inode we want in
> >> most cases I don't think we can be absolutely certain that it is. Maybe
> >> there's some way we could walk the list and be sure we've found the
> >> right inode, but I'm not seeing it.
> >
> > I guess we could do something like this (note that most of the changes
> > here are just to give a version of blkdev_get_by_path which takes a
> > struct path * so that the filename lookup doesn't have to be done
> > twice). Basically add a new hook that informs the security module of the
> > inode for the backing device file passed to mount and call that from
> > mount_bdev. The security module could grab a reference to the inode and
> > stash it away.
> >
> > Something else to note is that, as I have it here, the hook would end up
> > getting called for every mount of a given block device, not just the
> > first. So it's possible the security module could see the hook called a
> > second time with a different inode that has a different label. The hook
> > could be changed to return int if you wanted to have the opportunity to
> > reject such mounts.
>
> I'm not comfortable with this approach due to the aliasing/ambiguity you
> mention, as well as being unsure as to whether we truly want to label it
> the same as the backing block device (we certainly do not do that for
> normal mounts). Was also expecting the vfs folks to veto this patch but
> haven't seen that yet.

Yeah, I wasn't necessarily suggesting that this was a _good_ way to go,
only that I couldn't find a workable alternative.

> For now, how about if we just do this to compute the mountpoint label
> for SELinux:
> rc = security_transition_sid(current_sid(), current_sid(),
> SECCLASS_FILE, NULL, &sbsec->mntpoint_sid);
> if (rc)
> goto out;
>
> This will turn the current task context into a form suitable for a file
> object, while simultaneously allowing the policy writer to specify a
> different label for the files through policy transition rules if desired.

Great, I'll incorporate this. Thanks!

Seth