linux-kernel - Re: [PATCH 2/2] RFC: selinux: sysctl: fix selinux labeling broken by last patch

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Date:	Mon, 31 Jan 2011 09:21:20 -0500
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Lucian Adrian Grijincu <lucian.grijincu@...il.com>
Cc:	James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...isplace.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <dchinner@...hat.com>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	selinux <selinux@...ho.nsa.gov>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 2/2] RFC: selinux: sysctl: fix selinux labeling broken
 by last patch

On Mon, 2011-01-31 at 16:14 +0200, Lucian Adrian Grijincu wrote:
> On Mon, Jan 31, 2011 at 3:59 PM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
> > - Don't remove the IS_PRIVATE() test from inode_has_perm(), as other
> > inodes beyond just the /proc/sys ones are marked with that flag
> > (original usage was for reiserfs xattr inodes).
> 
> 
> Are you sure? I believe it was added here:
> 
>     [PATCH] selinux: enhance selinux to always ignore private inodes
> 
>     Hmmm...turns out to not be quite enough, as the /proc/sys inodes
> aren't truly
>     private to the fs, so we can run into them in a variety of security hooks
>     beyond just the inode hooks, such as security_file_permission (when reading
>     and writing them via the vfs helpers), security_sb_mount (when
> mounting other
>     filesystems on directories in proc like binfmt_misc), and deeper within the
>     security module itself (as in flush_unauthorized_files upon
> inheritance across
>     execve).  So I think we have to add an IS_PRIVATE() guard within SELinux, as
>     below.  Note however that the use of the private flag here could
> be confusing,
>     as these inodes are _not_ private to the fs, are exposed to userspace, and
>     security modules must implement the sysctl hook to get any access
> control over
>     them.
> 
> 
> http://thread.gmane.org/gmane.comp.security.selinux/341/focus=519
> 
> 
> In my patch I don't care about IS_PRIVATE, because I don't mark proc
> inodes as PRIVATE any more.
> 
> 
> This patch added S_ISPRIVATE to proc inodes:
>     [PATCH] sysctl: hide the sysctl proc inodes from selinux
>     86a71dbd3e81e8870d0f0e56b87875f57e58222b
> 
> This one added the IS_PRIVATE check:
>     [PATCH] selinux: enhance selinux to always ignore private inodes
>     bbaca6c2e7ef0f663bc31be4dad7cf530f6c4962
> 
> 
> I'll remove the check from my patch if you say it's used in other
> places too, but the original usage does not seem to be "for reiserfs
> xattr inodes".

Ok, my mistake - you can leave that part alone.  The original usage of
the S_PRIVATE flag was for reiserfs xattr inodes, but it appears that
you are correct that we don't need the IS_PRIVATE() guard within the
selinux code for that use case.

-- 
Stephen Smalley
National Security Agency

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