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

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Date:   Fri, 04 Aug 2023 12:00:39 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     David Howells <dhowells@...hat.com>,
        Paul Moore <paul@...l-moore.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Scott Mayhew <smayhew@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org
Subject: Re: [PATCH v6] vfs, security: Fix automount superblock LSM init
 problem, preventing NFS sb sharing

On Fri, 2023-08-04 at 16:22 +0100, David Howells wrote:
> David Howells <dhowells@...hat.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@...nel.org>