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linux-kernel: Re: Ответ: VFS, NFS security bug? Should CAP_

Re: Ответ: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS_MASK?

From: Serge E. Hallyn <serue_at_nospam>
Date: Tue Mar 17 2009 - 18:23:35 GMT
To: Stephen Smalley <>

Quoting Stephen Smalley (
> On Tue, 2009-03-17 at 12:39 -0500, Serge E. Hallyn wrote:
> > Quoting Stephen Smalley (
> > > > So do you think it makes sense to have CAP_MAC_ADMIN and CAP_FOWNER
> > > > in CAP_FS_MASK? In other words are you objecting to CAP_SYS_ADMIN
> > > > because of all of its other implications, or because you disagree
> > > > that labels for security modules should be treated as mere fs data
> > > > here?
> > >
> > > For CAP_FOWNER, yes (and it is already there). CAP_MAC_ADMIN is less
> >
> > Sorry, I meant CAP_SETFCAP. Should it be added?
> Sure - it is only used for filesystem operations.

Ok, so then:

> > > ideal as it isn't clearly tied to filesystem accesses, and likewise for
> > > CAP_MAC_OVERRIDE (but that one is included in CAP_FS_MASK already).
> >
> > So it is. I didn't realize that.
> >
> > > Ideally the capability space would be partitioned into capabilities that
> > > affect filesystem accesses and the rest so that setfsuid() would yield
> > > the expected behavior of only affecting filesystem access.
> > > CAP_SYS_ADMIN is even less suitable due to its pervasive use outside of
> > > the filesystem. So that's the first concern.
> > >
> > > The second one is that we don't want CAP_SYS_ADMIN (or CAP_MAC_ADMIN) to
> > > be required when setting SELinux labels. Only the SELinux permission
> > > checks should govern setting those labels (aside from the usual DAC
> > > ownership || CAP_FOWNER check).
> >
> > So if a non-selinux kernel is booted, then you think only the usual
> > DAC checks should be required to set selinux labels?
> I'm talking about the dumb NFS server case (non-SELinux NFS server
> providing label and data storage to SELinux clients, MAC enforcement
> handled client-side). But we aren't there yet, so I don't think we have
> to worry about it right now.

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?

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?

(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. thanks,
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