linux-kernel - Re: [PATCH 2/5] security,overlayfs: Provide security hook for copy up of xattrs for overlay file

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Date:	Tue, 5 Jul 2016 17:15:02 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	miklos@...redi.hu, sds@...ho.nsa.gov, linux-kernel@...r.kernel.org,
	linux-unionfs@...r.kernel.org,
	linux-security-module@...r.kernel.org, dwalsh@...hat.com,
	dhowells@...hat.com, pmoore@...hat.com, viro@...IV.linux.org.uk,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/5] security,overlayfs: Provide security hook for copy
 up of xattrs for overlay file

On Tue, Jul 05, 2016 at 01:22:22PM -0700, Casey Schaufler wrote:
> On 7/5/2016 8:50 AM, Vivek Goyal wrote:
> > Provide a security hook which is called when xattrs of a file are being
> > copied up. This hook is called once for each xattr and one can either
> > accept or reject xattr. If 0 is returned, xattr will be copied up, if 1
> > is returned, xattr will not be copied up and if negative error code
> > is returned, copy up will be aborted.
> >
> > In SELinux, label of lower file is not copied up. File already has been
> > set with right label at the time of creation and we don't want to overwrite
> > that label.
> >
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > ---
> >  fs/overlayfs/copy_up.c    |  8 ++++++++
> >  include/linux/lsm_hooks.h | 13 +++++++++++++
> >  include/linux/security.h  | 10 ++++++++++
> >  security/security.c       |  9 +++++++++
> >  security/selinux/hooks.c  | 14 ++++++++++++++
> >  5 files changed, 54 insertions(+)
> >
> > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > index 90dc362..2c31938 100644
> > --- a/fs/overlayfs/copy_up.c
> > +++ b/fs/overlayfs/copy_up.c
> > @@ -103,6 +103,14 @@ retry:
> >  			goto retry;
> >  		}
> >  
> > +		error = security_inode_copy_up_xattr(old, new,
> > +						     name, value, size);
> > +		if (error < 0)
> > +			break;
> > +		if (error == 1) {
> > +			error = 0;
> > +			continue; /* Discard */
> > +		}
> >  		error = vfs_setxattr(new, name, value, size, 0);
> >  		if (error)
> >  			break;
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index fcde9b9..2a8ee8c 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -412,6 +412,16 @@
> >   *	@src indicates the union dentry of file that is being copied up.
> >   *	@old indicates the pointer to old_cred returned to caller.
> >   *	Returns 0 on success or a negative error code on error.
> > + * @inode_copy_up_xattr:
> > + *	Filter the xattrs being copied up when a unioned file is copied
> > + *	up from a lower layer to the union/overlay layer.
> > + *	@src indicates the file that is being copied up.
> > + *	@dst indicates the file that has being created by the copy up.
> > + *	@name indicates the name of the xattr.
> > + *	@value, @size indicate the payload of the xattr.
> > + *	Returns 0 to accept the xattr, 1 to discard the xattr or a negative
> > + *	error code to abort the copy up. Note that the caller is responsible
> > + *	for reading and writing the xattrs as this hook is merely a filter.
> 
> The return should be -EOPNOTSUPP from security modules that don't
> support the attribute "name". This will make it possible to support
> multiple modules that provide attributes. (patches pending)

Hmm.., Sorry I did not understand this one. 

So all modules will not understand all xattrs. So if they start returning
-EOPNOTSUPP, then as per current implementation, copy up operation will
be aborted. 

Current implementation relies on that a security module, returns 0 if
every thing is "name" xattr should be copied up or lsm does not care.
Negative error code is returned only if something is wrong. Given every
lsm will not understand/care about all the xattrs, we can't return 
error code if lsm does not own/understand the "name". In fact
call_int_hook() will bail out the very first time negative error code
is returned. 

IOW, current implementation will work with multiple modules providing
implementation for same hook as long as module returns 0 for the xattrs
it does not understand. 

I guess I am missing something. Can you please elaborate a little more.

> 
> If the only use to which this hook is put is to identify attributes
> that should be discarded it's unnecessary overhead to pass the
> parameters that are never used.

Ok, I will get rid of extra parameters. If somebody needs these, it can
be added later.

Vivek