Re: [PATCH] keys: Make the KEY_NEED_* perms an enum rather than a mask

[Stephen Smalley]

On Wed, May 13, 2020 at 7:13 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Stephen Smalley <stephen.smalley.work@xxxxxxxxx> wrote:
>
> > >  (3) An override due to CAP_SYS_ADMIN.
> >
> > CAP_SYS_ADMIN should never skip SELinux checking.  Even for Smack,
> > there is a separate capability (CAP_MAC_ADMIN) for that purpose.
>
> The LSM doesn't get consulted at the moment.  With this patch, it will get
> consulted.
>
> > >  (4) An override due to an instantiation token being present.
> >
> > Not sure what this means but again we shouldn't skip SELinux checking
> > based on mere possession of an object capability (not a POSIX
> > capability).
>
> The kernel has delegated the instantiation of a key to the calling process and
> has given it a temporary key of type ".request_key_auth" which it has put into
> force with keyctl(KEYCTL_ASSUME_AUTHORITY).
>
> This authorisation token grants the caller the ability to (a) perform
> operations on the key it wouldn't otherwise have permission to do, (b) use the
> key instantiation keyctls and (c) temporarily search the keyrings of the
> caller of request_key() using the creds of that caller and to read/use the
> keys found therein if the caller was permitted to do so.
>
> > It would be better if the permission indicated the actual operation
> > (e.g. KEY_NEED_INVALIDATE_SPECIAL), and the decision whether to permit
> > CAP_SYS_ADMIN processes to override was left to the security modules.
> > SELinux doesn't automatically allow CAP_SYS_ADMIN processes to do
> > everything.
>
> These individual permissions don't exist yet.  I have an ACL patchset that
> allows me to add a greater range - though there's issues with SELinux there
> also.
>
> Also, the keyrings are specially marked to say that the sysadmin is allowed to
> flush them at the moment - but that can go away with the ACL stuff.
>
> > > +       switch (need_perm) {
> > > +       case KEY_NEED_UNLINK:
> > > +       case KEY_SYSADMIN_OVERRIDE:
> > > +       case KEY_AUTHTOKEN_OVERRIDE:
> > > +       case KEY_DEFER_PERM_CHECK:
> > >                 return 0;
> >
> > We really shouldn't be skipping any/all checking on CAP_SYS_ADMIN or
> > an AUTHTOKEN; those should still be subject to MAC policy.
>
> I'm not sure how to do that.
>
> Note that KEY_NEED_UNLINK *must not* be overruled by the MAC policy.  The
> value is only there because lookup_user_key() requires something to be put
> into that parameter - it's more of a courtesy thing, I suppose.
>
> Why should AUTHTOKEN be subject to MAC policy?  The kernel has told the
> process to go and instantiate a key.  It shouldn't really then turn around and
> tell the process "oh, but you're not actually allowed to do that".

On what basis did the kernel authorize the process to instantiate the
key?  At what point did a security module get involved in the decision
as to what process(es) are authorize to instantiate a key,
particularly for a process with a different credential/security
context?