linux-kernel - Re: Are path-based LSM hooks called from the wrong places?

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Message-ID: <20090326155357.GS28946@ZenIV.linux.org.uk>
Date:	Thu, 26 Mar 2009 15:53:57 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	David Howells <dhowells@...hat.com>
Cc:	Kentaro Takeda <takedakn@...data.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Toshiharu Harada <haradats@...data.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: Are path-based LSM hooks called from the wrong places?

On Wed, Mar 25, 2009 at 04:14:13PM +0000, David Howells wrote:
> 
> Hi Kentaro,
> 
> I've just been looking at some of the VFS syscall routines, such as
> notify_change(), with an eye to calling it from FS-Cache to grow a file.  I
> see that whilst notify_change() calls the inode-based LSM hooks (as drive
> SELinux), it doesn't call the path-based LSM hooks (as drive other security
> modules).  It leaves that to the callers, such as do_sys_ftruncate().
> 
> I see that vfs_mkdir(), for example, is similar, in that vfs_mkdir() - which
> I'm calling from FS-Cache - invokes the inode-based LSM hooks, but it bypasses
> the path-based LSM hooks as those are called from sys_mkdir().
> 
> It would appear that path-based LSM hooks may well be being called from the
> wrong places.  They were added in:
> 
> 	commit be6d3e56a6b9b3a4ee44a0685e39e595073c6f0d
> 	Author: Kentaro Takeda <takedakn@...data.co.jp>
> 	Date:   Wed Dec 17 13:24:15 2008 +0900
> 
> 	    introduce new LSM hooks where vfsmount is available.
> 
> 	    Add new LSM hooks for path-based checks.  Call them on directory-modifying
> 	    operations at the points where we still know the vfsmount involved.
> 
> 	    Signed-off-by: Kentaro Takeda <takedakn@...data.co.jp>
> 	    Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> 	    Signed-off-by: Toshiharu Harada <haradats@...data.co.jp>
> 	    Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> 
> Using sys_mkdir() and suchlike directly from within the kernel would add a lot
> of overhead as I'd have to generate a full pathname for each call, whereas
> vfs_mkdir() or notify_change() allows me to start from an inode I already
> have.

If you start from inode (or dentry, for that matter), you don't *have*
a pathname at all.  The real question is, do you want these checks to
apply and if you do - which path do you want to use (esp. if you have
multiple namespaces)?
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