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 14:34:43 -0700
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	Vivek Goyal <vgoyal@...hat.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 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.

>
> 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
>