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

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Date:   Fri, 21 Jun 2019 13:18:03 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Dave Young <dyoung@...hat.com>
Cc:     James Morris <jmorris@...ei.org>, Jiri Bohac <jbohac@...e.cz>,
        Linux API <linux-api@...r.kernel.org>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH V31 07/25] kexec_file: Restrict at runtime if the kernel
 is locked down

On Thu, Jun 20, 2019 at 11:43 PM Dave Young <dyoung@...hat.com> wrote:
>
> On 03/26/19 at 11:27am, Matthew Garrett wrote:
> > From: Jiri Bohac <jbohac@...e.cz>
> >
> > 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@...e.cz>
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > Signed-off-by: Matthew Garrett <mjg59@...gle.com>
> > Reviewed-by: Jiri Bohac <jbohac@...e.cz>
> > cc: kexec@...ts.infradead.org
> > ---
> >  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.