linux-kernel - Re: [PATCH] Security/sysfs: Enable security xattrs to be set on sysfs files, directories, and symlinks.

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Date:	Thu, 09 Jul 2009 10:05:06 -0400
From:	"David P. Quigley" <dpquigl@...ho.nsa.gov>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	jmorris@...ei.org, gregkh@...e.de, sds@...ho.nsa.gov,
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Security/sysfs: Enable security xattrs to be set on
 sysfs files, directories, and symlinks.

On Wed, 2009-07-08 at 18:44 -0700, Casey Schaufler wrote:
> David P. Quigley wrote:
> > This patch adds a setxattr handler to the file, directory, and symlink
> > inode_operations structures for sysfs. This handler uses two new LSM hooks. The
> > first hook takes the xattr name and value and turns the context into a secid.
> > This is embedded into the sysfs_dirent structure so it remains persistent even
> > if the inode structures are evicted from the cache. The second hook allows for
> > the secid to be taken from the sysfs_dirent and be pushed into the inode
> > structure as the actual secid for the inode.
> >   
> 
> Nacked-by: Casey Schaufler <casey@...aufler-ca.com>
> 
> I'm all for sysfs supporting xattrs.
> 
> I am completely opposed to secids as file system metadata.
> 
> What do you get when you do an ls -Z?
> 
> An LSM must not be beholden to exposing transient internal
> representations of security data to userspace, which is what
> you're doing here. An LSM gets to decide what the security
> information it maintains looks like by defining a security blob.
> 
> If you want this in, implement xattrs in sysfs for real. Smack
> depends on the existing, published, and supported xattr interfaces
> for dealing with getting and setting the values. Not secids.
> Smack maintains secids because labeled networking and audit require
> them, and they got there first.
> 
> 

So are you proposing that we embed a variable length string in the
sysfs_dirent structure because that sounds completely silly. It seems
completely reasonable here to take the blob coming in and have the LSM
turn it into a handle that is efficiently referenced by the
sysfs_dirent. The problem here is that sysfs entries have no backing
store at all which means everything we do will have to be added to
sysfs_dirent. I'm pretty sure we don't want to be doing lifecycle
management on strings inside this structure considering the only other
string I see is marked const. If you have a better way of doing this I'm
interested in hearing it but it doesn't seem reasonable to be storing
the xattr itself in the sysfs_dirent. I'd like to hear what Greg thinks
about that.

Dave

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