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. But we already have the current->creds. Can't that be used to control the permission checking done by the callback? Thanks, Miklos -- 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
