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

From: Stephen Smalley <sds_at_nospam>
Date: Tue Mar 17 2009 - 17:52:25 GMT
To: "Serge E. Hallyn" <serue@us.ibm.com>


On Tue, 2009-03-17 at 12:39 -0500, Serge E. Hallyn wrote:
> Quoting Stephen Smalley (sds@tycho.nsa.gov):
> > > 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.

> > 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.

> Or is this assuming EOPNOTSUPP were returned for setting
> security.selinux xattrs in such a kernel?
>
> > > > uses CAP_SYS_ADMIN to control setting of its own attributes:
> > > > - SELinux applies a DAC check and its own set of MAC file permission
> > > > checks,
> > > > - Smack applies CAP_MAC_ADMIN,
> > > > - Capabilities applies CAP_SETFCAP.
> > > >
> > > > Checking CAP_SYS_ADMIN was really just a fallback to prevent unchecked
> > > > setting of attributes in the no-LSM case. It might make more sense to
> > > > return EOPNOTSUPP for any attributes unknown to the enabled security
> > >
> > > I suspect that would create a LOT of bug reports. Would requiring
> > > CAP_MAC_ADMIN seem reasonable?
> >
> > It would narrow the scope a bit more, but it still isn't ideal.
> >
> > > > module and require you to enable the desired module before setting the
> > > > attributes these days.
> > > > http://marc.info/?t=107428809400002&r=1&w=2
> > > >
> > > > I don't think this will make any difference for labeled NFS at present,
> > > > as the current labeled NFS patches only export the MAC label attribute
> > > > if the server has the MAC model enabled. So CAP_SYS_ADMIN won't get
> > > > checked regardless.
> > > >
> > > > Trusted namespace is another case where CAP_SYS_ADMIN check is applied
> > > > on file operations.
> > >
> > > Which seems like all the more reason why CAP_SYS_ADMIN would need to
> > > be added to the CAP_FS_MASK. Or do you mean that check should also be
> > > changed for something else? (CAP_MAC_ADMIN, or some new CAP_FS_XATTR?)
> >
> > I'd favor changing it to a new capability. We have CAP_SETFCAP for
> > setting file capabilities; why not have CAP_SETTRUSTED for setting
> > attributes in the trusted namespace? Then adding it to CAP_FS_MASK has
> > no further side effects beyond controlling filesystem accesses.
>
> Does anyone know what the trusted xattrs are used for?

Not beyond what attr(5) says about them. -- Stephen Smalley National Security Agency -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html