linux-kernel - Re: [PATCH v4 17/21] capabilities: Allow privileged user in s_user

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Date:	Wed, 27 Apr 2016 17:22:02 +1000 (AEST)
From:	James Morris <jmorris@...ei.org>
To:	Seth Forshee <seth.forshee@...onical.com>
cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	James Morris <james.l.morris@...cle.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Miklos Szeredi <mszeredi@...hat.com>,
	Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...hat.com, linux-raid@...r.kernel.org,
	linux-mtd@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	cgroups@...r.kernel.org
Subject: Re: [PATCH v4 17/21] capabilities: Allow privileged user in s_user_ns
 to set security.* xattrs

On Tue, 26 Apr 2016, Seth Forshee wrote:

> A privileged user in s_user_ns will generally have the ability to
> manipulate the backing store and insert security.* xattrs into
> the filesystem directly. Therefore the kernel must be prepared to
> handle these xattrs from unprivileged mounts, and it makes little
> sense for commoncap to prevent writing these xattrs to the
> filesystem. The capability and LSM code have already been updated
> to appropriately handle xattrs from unprivileged mounts, so it
> is safe to loosen this restriction on setting xattrs.
> 
> The exception to this logic is that writing xattrs to a mounted
> filesystem may also cause the LSM inode_post_setxattr or
> inode_setsecurity callbacks to be invoked. SELinux will deny the
> xattr update by virtue of applying mountpoint labeling to
> unprivileged userns mounts, and Smack will deny the writes for
> any user without global CAP_MAC_ADMIN, so loosening the
> capability check in commoncap is safe in this respect as well.
> 
> Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> Acked-by: Serge Hallyn <serge.hallyn@...onical.com>


Acked-by: James Morris <james.l.morris@...cle.com>


-- 
James Morris
<jmorris@...ei.org>