Date: Fri, 26 May 2017 11:28:21 -0400
From: Scott Mayhew <smayhew@redhat.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: selinux@tycho.nsa.gov, linux-nfs@vger.kernel.org,
        Paul Moore <paul@paul-moore.com>,
        Eric Paris <eparis@parisplace.org>,
        Trond Myklebust <trondmy@primarydata.com>,
        "J . Bruce Fields" <bfields@fieldses.org>
Subject: Re: [PATCH RFC 2/2] nfs: update labeling behavior on a superblock
 when submounting
Message-ID: <20170526152821.xgyidg7qebdwbm6f@tonberry.usersys.redhat.com>
References: <20170404232646.GB24146@parsley.fieldses.org>
 <20170525210754.24265-1-smayhew@redhat.com>
 <20170525210754.24265-3-smayhew@redhat.com>
 <1495808645.12091.10.camel@tycho.nsa.gov>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
In-Reply-To: <1495808645.12091.10.camel@tycho.nsa.gov>
Sender: linux-nfs-owner@vger.kernel.org

On Fri, 26 May 2017, Stephen Smalley wrote:

> On Thu, 2017-05-25 at 17:07 -0400, Scott Mayhew wrote:
> > When the client traverses from filesystem exported without the
> > "security_label" option to one exported with the "security_label"
> > option, it needs to pass SECURITY_LSM_NATIVE_LABELS to
> > security_sb_set_mnt_opts() so that the new superblock has SBLABEL_MNT
> > set in its security mount options.??Otherwise, attempts to set
> > security
> > labels via setxattr over NFSv4.2 will fail.
> > 
> > Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> > ---
> > ?fs/nfs/super.c | 23 ++++++++++++++++++++++-
> > ?1 file changed, 22 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> > index 2f3822a..d7a3b89 100644
> > --- a/fs/nfs/super.c
> > +++ b/fs/nfs/super.c
> > @@ -2544,10 +2544,31 @@ EXPORT_SYMBOL_GPL(nfs_set_sb_security);
> > ?int nfs_clone_sb_security(struct super_block *s, struct dentry
> > *mntroot,
> > ?			??struct nfs_mount_info *mount_info)
> > ?{
> > +	int error;
> > +	unsigned long kflags = 0, kflags_out = 0;
> > +	struct security_mnt_opts opts;
> > +
> > ?	/* clone any lsm security options from the parent to the new
> > sb */
> > ?	if (d_inode(mntroot)->i_op != NFS_SB(s)->nfs_client-
> > >rpc_ops->dir_inode_ops)
> > ?		return -ESTALE;
> > -	return security_sb_clone_mnt_opts(mount_info->cloned->sb,
> > s);
> > +	error = security_sb_clone_mnt_opts(mount_info->cloned->sb,
> > s);
> > +	if (error)
> > +		goto err;
> > +
> > +	if (NFS_SB(s)->caps & NFS_CAP_SECURITY_LABEL &&
> > +		!(NFS_SB(mount_info->cloned->sb)->caps &
> > NFS_CAP_SECURITY_LABEL)) {
> > +		memset(&opts, 0, sizeof(opts));
> > +		kflags |= SECURITY_LSM_NATIVE_LABELS;
> > +
> > +		error = security_sb_set_mnt_opts(s, &opts, kflags,
> > &kflags_out);
> > +		if (error)
> > +			goto err;
> > +
> > +		if (!(kflags_out & SECURITY_LSM_NATIVE_LABELS))
> > +			NFS_SB(s)->caps &= ~NFS_CAP_SECURITY_LABEL;
> > +	}
> > +err:
> > +	return error;
> > ?}
> > ?EXPORT_SYMBOL_GPL(nfs_clone_sb_security);
> 
> Could this clobber a context set via context= mount option?

Argh, yes I suppose it could.  In my first attempt to fix this, I added
a security_sb_get_mnt_opts() hook to get the original mount options and
then passed that along with the SECURITY_LSM_NATIVE_LABELS flag to
security_sb_set_mnt_opts().  When I saw that security_sb_set_mnt_opts()
wouldn't allow me to change a superblock that had already been
initialized, I got rid of the hook and added the check in patch 1...
maybe a combination of the two is needed?

Testing it again now, I'm not sure the context= mount option is working
correctly with the latest kernel.

-Scott