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

[Christian Göttsche]

On Wed, 24 Nov 2021 at 19:16, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 24, 2021 at 11:33:11AM -0600, Serge E. Hallyn wrote:
> > 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@xxxxxxxxxx>
> > > > >
> > > > > 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.
>

I proposed an early prototype at
https://patchwork.kernel.org/project/selinux/patch/20211116112437.43412-1-cgzones@xxxxxxxxxxxxxx/

> Sure, feel free to work on that and submit it, but for now, this change
> is needed.
>

I would argue this change is not necessary since the actual syscall
still succeeds as this is only an informative avc denial message about
a failed capability check. But this ship has sailed...

> thanks,
>
> greg k-h