Re: [PATCH 5/5] overlayfs: Use vfs_getxattr_noperm() for real inode

[Vivek Goyal <vgoyal@redhat.com>]

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@redhat.com> 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.

Vivek