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

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Date:	Thu, 26 Mar 2009 16:14:49 +0900
From:	Kentaro Takeda <takedakn@...data.co.jp>
To:	dhowells@...hat.com
CC:	penguin-kernel@...ove.SAKURA.ne.jp, haradats@...data.co.jp,
	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: Are path-based LSM hooks called from the wrong places?

David,
thanks for your attention. :)

David Howells wrote:
> 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>
Well, my understanding is that your question is related to two aspects of
the new LSM hooks: location and coverage.

(about location)
This patch introduced new LSM hooks outside vfs helper functions because
Al had adviced us that vfs helper functions should remain vfsmount-unaware.

(about coverage)
This patch is not intended to implement the equivalent of security_inode_*
hooks. This patch is for pathname-based MAC, especially for TOMOYO 
in kernel 2.6.30.

> 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.
Callers of sys_mkdir() and suchlike directly from within the kernel don't need
to generate a full pathname. Name based LSM will generate a full pathname from
dentry and vfsmount. There is no getname() nor path_walk() call.

Regards,

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