Date:	Wed, 6 Jul 2016 13:09:00 -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 02:34:43PM -0700, Casey Schaufler wrote:
> On 7/5/2016 2:15 PM, Vivek Goyal wrote:
> > 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. 
> 
> Yes, the infrastructure code will have to change to deal with the
> tri-state returns. That's also true of several other hooks.
> 
> > 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. 
> 
> There have to be four states. I own this attribute, and want you
> to use it. I own this attribute and I want you to ignore it. I don't
> own this attribute. I own this attribute and something went terribly
> wrong, such as running out of memory.

Ok, so we have 3 states currently and we should have four.

I own this attribute and want you to use it ---> Return 0
I own this attribute and want you to ignore it --> Return 1
I don't own this attribute --> -EOPNOTSUPP
Something went terribly wrong --> Negative error code.

I can modify call_int_hook() to continue if -EOPNOTSUPP is returned. And
if none of the LSMs claimed xattr, caller will get -EOPNOTSUPP.

But what is caller supposed to do with it. There might be xattrs which
are just user data (user.foo) and aborting copying up will not make sense.
That means caller will continue to copy up anyway and treat -EOPNOTSUPP
as success.

IOW, What are we going to gain by introducing this extra state when none
of the LSMs claims to know about the xattr name passed in.

Vivek

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