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

[Greg Kroah-Hartman]

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