Subject: Re: =?UTF-8?Q?=D0=9E=D1=82=D0=B2=D0=B5=D1=82=3A?= VFS, NFS
 security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to
 CAP_FS_MASK?
From: Stephen Smalley <sds@tycho.nsa.gov>
To: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: Igor Zhbanov <izh1979@gmail.com>, "J. Bruce Fields" <bfields@fieldses.org>,
       Michael Kerrisk <mtk.manpages@gmail.com>, linux-kernel@vger.kernel.org,
       viro@zeniv.linux.org.uk, neilb@suse.de, Trond.Myklebust@netapp.com,
       David Howells <dhowells@redhat.com>, Andrew Morgan <morgan@kernel.org>,
       James Morris <jmorris@namei.org>, linux-security-module@vger.kernel.org,
       SELinux <selinux@tycho.nsa.gov>
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Content-Type: text/plain
Organization: National Security Agency
Date: Wed, 18 Mar 2009 12:21:27 -0400
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On Tue, 2009-03-17 at 13:23 -0500, Serge E. Hallyn wrote:
> But in cap_inode_setxattr, any security.* xattrs are controlled by
> CAP_SYS_ADMIN.  So do you think that this should be changed to a
> CAP_XATTR_SECURITY capability which can be added to CAP_FS_MASK?

I think that would be preferable to CAP_SYS_ADMIN, yes.

> Or do you think it's ok that fsuid=0 does not allow you to set
> security.selinux (or security.SMACK64, etc) xattrs when selinux is
> not compiled in?

Just to be clear, at present fsuid is irrelevant to setting the
security.* xattrs since it doesn't affect the CAP_SYS_ADMIN capability
at all, so it all depends on the initial capability state prior to using
setfsuid(), typically the full capability set.

> (You may have already answered this with your EOPNOTSUPP comment, but
> I want to make sure I understand right)
> 
> > > Does anyone know what the trusted xattrs are used for?
> > 
> > Not beyond what attr(5) says about them.
> 
> Well, if attr(5) says CAP_SYS_ADMIN being needed is the very
> thing that defines these xattrs, then changing that seems a
> bigger deal.  That really does seem akin to changing kernel-user
> API.

Perhaps, although it isn't clear that this API is in use by anyone or in
use in a way that would actually distinguish based on individual
capability.

But if you were to add CAP_SYS_ADMIN to CAP_FS_MASK in order to ensure
that setfsuid() does in fact affect all filesystem accesses, how much
meaningful difference remains between fsuid==0 and euid==0?  It
obviously takes you far afield of only affecting filesystem accesses.

-- 
Stephen Smalley
National Security Agency

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