linux-kernel - Re: Ответ: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS

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Date:	Wed, 18 Mar 2009 12:21:27 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Igor Zhbanov <izh1979@...il.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	neilb@...e.de, Trond.Myklebust@...app.com,
	David Howells <dhowells@...hat.com>,
	Andrew Morgan <morgan@...nel.org>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	SELinux <selinux@...ho.nsa.gov>
Subject: Re: Ответ: VFS, NFS
 security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to
 CAP_FS_MASK?

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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