Re: [PATCH V31 07/25] kexec_file: Restrict at runtime if the kernel is locked down

[Dave Young]

On 06/21/19 at 01:18pm, Matthew Garrett wrote:
> On Thu, Jun 20, 2019 at 11:43 PM Dave Young <dyoung@xxxxxxxxxx> wrote:
> >
> > On 03/26/19 at 11:27am, Matthew Garrett wrote:
> > > From: Jiri Bohac <jbohac@xxxxxxx>
> > >
> > > When KEXEC_SIG is not enabled, kernel should not load images through
> > > kexec_file systemcall if the kernel is locked down.
> > >
> > > [Modified by David Howells to fit with modifications to the previous patch
> > >  and to return -EPERM if the kernel is locked down for consistency with
> > >  other lockdowns. Modified by Matthew Garrett to remove the IMA
> > >  integration, which will be replaced by integrating with the IMA
> > >  architecture policy patches.]
> > >
> > > Signed-off-by: Jiri Bohac <jbohac@xxxxxxx>
> > > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> > > Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx>
> > > Reviewed-by: Jiri Bohac <jbohac@xxxxxxx>
> > > cc: kexec@xxxxxxxxxxxxxxxxxxx
> > > ---
> > >  kernel/kexec_file.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > > index 67f3a866eabe..a1cc37c8b43b 100644
> > > --- a/kernel/kexec_file.c
> > > +++ b/kernel/kexec_file.c
> > > @@ -239,6 +239,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
> > >               }
> > >
> > >               ret = 0;
> > > +
> > > +             if (kernel_is_locked_down(reason, LOCKDOWN_INTEGRITY)) {
> > > +                     ret = -EPERM;
> > > +                     goto out;
> > > +             }
> > > +
> >
> > Checking here is late, it would be good to move the check to earlier
> > code around below code:
> >         /* We only trust the superuser with rebooting the system. */
> >         if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> >                 return -EPERM;
> 
> I don't think so - we want it to be possible to load images if they
> have a valid signature.

I know it works like this way because of the previous patch.  But from
the patch log "When KEXEC_SIG is not enabled, kernel should not load
images", it is simple to check it early for !IS_ENABLED(CONFIG_KEXEC_SIG) && 
kernel_is_locked_down(reason, LOCKDOWN_INTEGRITY)  instead of depending
on the late code to verify signature.  In that way, easier to
understand the logic, no?

Thanks
Dave