linux-kernel - Re: [TOMOYO #8 (2.6.25-mm1) 1/7] Introduce new LSM hooks.

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Date:	Thu, 1 May 2008 08:47:16 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	chrisw@...s-sol.org, viro@...IV.linux.org.uk,
	akpm@...ux-foundation.org, linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	takedakn@...data.co.jp, haradats@...data.co.jp
Subject: Re: [TOMOYO #8 (2.6.25-mm1) 1/7] Introduce new LSM hooks.

* Tetsuo Handa (penguin-kernel@...ove.SAKURA.ne.jp) wrote:
> The MAY_WRITE flag is not passed to security_inode_permission()
> if security_inode_permission() is called from __open_namei_create().
> Since TOMOYO Linux doesn't check MAY_READ/MAY_WRITE permissions for individual
> read()/write() requests, the permission checks at open() time (i.e. may_open())
> is the only chance to check MAY_WRITE flag. If I can't check MAY_WRITE flag
> here, TOMOYO Linux can't control open(O_WRONLY | O_CREATE | O_EXCL).
> 
> Also, the O_TRUNC flag is not passed to security_inode_permission() because
> vfs_permission() receives only MAY_READ/MAY_WRITE/MAY_APPEND flags, but
> I have to check O_TRUNC flag before do_truncate().
> 
> So, I inserted a new hook here so that this hook can check all
> MAY_READ/MAY_WRITE/O_APPEND/O_TRUNC flags together in a single place.

The reason I ask is because it doesn't check.  It only checks O_APPEND,
but that's already passed in (MAY_APPEND).  So AFAICT, it's only O_TRUNC
that you are trying to special case.  But in that case...all that is
being asked for is MAY_WRITE permission.  Anything else doesn't make
sense, especially since that's all you get from the truncate(2) path.

<snip>
> This is an inevitable duplication since I want to do conventional checks
> (DAC checks and inode operation existence checks) before TOMOYO Linux's check.
> 
> By the way, Stephen Smalley thinks it is better to copy codes which is needed by
> pre_vfs_*() (i.e. may_create()/may_delete()/check_sticky()) into
> security/tomoyo/ directory and leave vfs_*() untouched rather than
> extract pre_vfs_*() from vfs_*() and call pre_vfs_*() from vfs_*().

I'm not sure he means literally copy.  Typically we take existing
functionality and make it externally usable.

Also, all the changes you make that are not in vfs_* helpers won't get
picked up by NFS.

thanks,
-chris
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