[PATCH v6] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Jeff Layton]

On Fri, 2023-08-04 at 16:22 +0100, David Howells wrote:
> David Howells <dhowells at redhat.com> wrote:
> 
> > IIRC, the issue is when you make a mount with an explicit context= setting and
> > make another mount from some way down the export tree that doesn't have an
> > explicit setting, e.g.:
> > 
> > 	mount carina:/ /mnt -o context=system_u:object_r:root_t:s0
> > 	mount carina:/nfs/scratch /mnt2
> > 
> > and then cause an automount to walk from one to the other:
> > 
> > 	stat /mnt/nfs/scratch/foo
> 
> Actually, the order there isn't quite right.  The problem is with this order:
> 
> 	# mount carina:/ /mnt -o context=system_u:object_r:root_t:s0
> 	# stat /mnt/nfs/scratch/bus
> 	  File: /mnt/nfs/scratch/bus
> 	  Size: 124160          Blocks: 248        IO Block: 1048576 regular file
> 	Device: 0,55    Inode: 131         Links: 1
> 	...
> 	# mount carina:/nfs/scratch /mnt2
> 	mount.nfs: /mnt2 is busy or already mounted or sharecache fail
> 
> with the error:
> 
> 	SELinux: mount invalid.  Same superblock, different security settings for (dev 0:52, type nfs4)
> 

That seems like the correct behavior to me. You tried to mount the same
mount with a different sec context. If you want that, then you need to
use -o nosharecache.

I'll send a v7 in a bit. 
-- 
Jeff Layton <jlayton at kernel.org>