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

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Jul 6, 2016 at 12:54 PM, Vivek Goyal <vgoyal@redhat.com> 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@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.

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.

Thanks,
Miklos