linux-kernel - Re: [patch] security: fix dummy xattr functions

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Date:	Wed, 02 Jul 2008 11:28:31 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jmorris@...ei.org
CC:	miklos@...redi.hu, jjohansen@...e.de, akpm@...ux-foundation.org,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, serue@...ibm.com, morgan@...nel.org
Subject: Re: [patch] security: fix dummy xattr functions

On Wed, 2 Jul 2008, James Morris wrote:
> On Wed, 2 Jul 2008, Miklos Szeredi wrote:
> 
> > So where do the dummy_ functions figure into this?  As I understand,
> > they are called whenever LSM is disabled, but the LSM doesn't define a
> > particular hook, so there's a default implementation.  Is that correct?
> 
> If LSM is disabled, nothing is called (the security hooks are optimized 
> away).

Right.  I meant to say "enabled", instead of "disabled" above :)

>  It's for when LSM is enabled, but there is either no LSM module 
> selected, or as fallbacks for hooks which are not implemented by an LSM 
> module.

Yes, we were thinking of the fallback case.  When falling back to the
default, that should be equivalent to the "no LSM" case, no?

Currently it's not.

> > If so, then in theory it is still theoretically possible that with
> > LSM+capabilities, the LSM doesn't explicitly stack inode_setxattr and
> > inode_removexattr, and so the dummy implementation should do that
> > instead.  What am I missing?
> 
> The LSM is responsible for performing this stacking (or not), depending on 
> which particular security models are desired.  It may, for example, not 
> want filesystem capabilities.
> 
> I guess it might be safer to force the LSM to override fs capabilities if 
> it doesn't want them, but I'd like to see what others think.

No, this patch is _not_ forcing anything is LSM defines its own
inode_{set,remove}xattr methods.  I agree, that should be decided by
the LSM.

The patch is just fixing the fallback dummy functions to be in sync
with the the disabled LSM case.

Miklos
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