[PATCH 5.10 130/154] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

[Serge E. Hallyn]

On Wed, Nov 24, 2021 at 04:31:22PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 24, 2021 at 04:22:50PM +0200, Jari Ruusu wrote:
> > Greg Kroah-Hartman wrote:
> > > From: Alistair Delva <adelva at google.com>
> > > 
> > > commit 94c4b4fd25e6c3763941bdec3ad54f2204afa992 upstream.
> >  [SNIP]
> > > --- a/block/ioprio.c
> > > +++ b/block/ioprio.c
> > > @@ -69,7 +69,14 @@ int ioprio_check_cap(int ioprio)
> > > 
> > >         switch (class) {
> > >                 case IOPRIO_CLASS_RT:
> > > -                       if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
> > > +                       /*
> > > +                        * Originally this only checked for CAP_SYS_ADMIN,
> > > +                        * which was implicitly allowed for pid 0 by security
> > > +                        * modules such as SELinux. Make sure we check
> > > +                        * CAP_SYS_ADMIN first to avoid a denial/avc for
> > > +                        * possibly missing CAP_SYS_NICE permission.
> > > +                        */
> > > +                       if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
> > >                                 return -EPERM;
> > >                         fallthrough;
> > >                         /* rt has prio field too */
> > 
> > What exactly is above patch trying to fix?
> > It does not change control flow at all, and added comment is misleading.
> 
> See the thread on the mailing list for what it does and why it is
> needed.
> 
> It does change the result when selinux is enabled.
> 
> thanks,
> 
> greg k-h

The case where we create a newer more fine grained capability which is a
sub-cap of a broader capability like CAP_SYS_ADMIN is analogous.  See
check_syslog_permissions() for instance.

So I think a helper like

int capable_either_or(int cap1, int cap2) {
	if (has_capability_noaudit(current, cap1))
		return 0;
	return capable(cap2);
}

might be worthwhile.