On Wed, Jul 06, 2016 at 04:58:37PM +0200, Miklos Szeredi wrote: > On Wed, Jul 6, 2016 at 12:54 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > On Wed, Jul 06, 2016 at 06:36:49AM +0200, Miklos Szeredi wrote: > >> On Tue, Jul 5, 2016 at 11:16 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > >> > On Tue, Jul 05, 2016 at 01:29:39PM -0700, Casey Schaufler wrote: > >> >> On 7/5/2016 8:50 AM, Vivek Goyal wrote: > >> >> > ovl_getxattr() currently uses vfs_getxattr() on realinode. This fails > >> >> > if mounter does not have DAC/MAC permission to access getxattr. > >> >> > > >> >> > Specifically this becomes a problem when selinux is trying to initialize > >> >> > overlay inode and does ->getxattr(overlay_inode). A task might trigger > >> >> > initialization of overlay inode and we will access real inode xattr in the > >> >> > context of mounter and if mounter does not have permissions, then inode > >> >> > selinux context initialization fails and inode is labeled as unlabeled_t. > >> >> > > >> >> > One way to deal with it is to let SELinux do getxattr checks both on > >> >> > overlay inode and underlying inode and overlay can call vfs_getxattr_noperm() > >> >> > to make sure when selinux is trying to initialize label on inode, it does > >> >> > not go through checks on lower levels and initialization is successful. > >> >> > And after inode initialization, SELinux will make sure task has getatttr > >> >> > permission. > >> >> > > >> >> > One issue with this approach is that it does not work for directories as > >> >> > d_real() returns the overlay dentry for directories and not the underlying > >> >> > directory dentry. > >> >> > > >> >> > Another way to deal with it to introduce another function pointer in > >> >> > inode_operations, say getxattr_noperm(), which is responsible to get > >> >> > xattr without any checks. SELinux initialization code will call this > >> >> > first if it is available on inode. So user space code path will call > >> >> > ->getxattr() and that will go through checks and SELinux internal > >> >> > initialization will call ->getxattr_noperm() and that will not > >> >> > go through checks. > >> >> > > >> >> > For now, I am just converting ovl_getxattr() to get xattr without > >> >> > any checks on underlying inode. That means it is possible for > >> >> > a task to get xattr of a file/dir on lower/upper through overlay mount > >> >> > while it is not possible outside overlay mount. > >> >> > > >> >> > If this is a major concern, I can look into implementing getxattr_noperm(). > >> >> > >> >> This is a major concern. > >> > > >> > Hmm.., In that case I will write patch to provide another inode operation > >> > getxattr_noperm() and a wrapper which falls back to getxattr() if noperm > >> > variant is not defined. That should take care of this issue. > >> > >> That's not going to fly. A slighly better, but still quite ugly > >> solution would be to add a "flags" arg to the current ->getxattr() > >> callback indicating whether the caller wants permission checking > >> inside the call or not. > >> > > > > Ok, will try that. > > > >> But we already have the current->creds. Can't that be used to control > >> the permission checking done by the callback? > > > > Sorry, did not get how to use current->creds to control permission > > checking. > > I'm not sure about the details either. But current->creds *is* the > context provided for the VFS and filesystems to check permissions. It > might make sense to use that to indicate to overlayfs that permission > should not be checked. That sounds like raising capabilities of task temporarily to do getxattr(). But AFAIK, there is no cap which will override SELinux checks. I am taking a step back re-thinking about the problem. - For context mounts this is not a problem at all as overlay inode will get its label from context= mount option and we will not call into ovl_getxattr(). - For non-context mounts this is a problem only if mounter is not privileged enough to do getattr. And that's not going to be a common case either. IOW, this does not look like a common case. And if getxattr() fails, SELinux already seems to mark inode as unlabeled_t. And my understanding is that task can't access unlabeled_t anyway, so there is no information leak. So for now, why not leave it as it is. Only side affect I seem to see is following warnings on console. SELinux: inode_doinit_with_dentry: getxattr returned 13 for dev=overlay ino=29147 This is for information purposes only and given getxattr() can fail in stacked configuration, I think we can change this to KERN_DEBUG instead of KERN_WARNING. Thanks Vivek -- 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
