[PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled

[Ricardo Ribalda]

Hi Philipp

Thanks for your review!

On Thu, 17 Nov 2022 at 16:07, Philipp Rudo <prudo at redhat.com> wrote:
>
> Hi Ricardo,
>
> all in all I think this patch makes sense. However, there is one point
> I don't like...
>
> On Mon, 14 Nov 2022 14:18:39 +0100
> Ricardo Ribalda <ribalda at chromium.org> wrote:
>
> > Create a new toogle that disables LINUX_REBOOT_CMD_KEXEC, reducing the
> > attack surface to a system.
> >
> > Without this toogle, an attacker can only reboot into a different kernel
> > if they can create a panic().
> >
> > Signed-off-by: Ricardo Ribalda <ribalda at chromium.org>
> >
> > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > index 97394bd9d065..25d019682d33 100644
> > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > @@ -462,6 +462,17 @@ altered.
> >  Generally used together with the `modules_disabled`_ sysctl.
> >
> >
> > +kexec_reboot_disabled
> > +=====================
> > +
> > +A toggle indicating if ``LINUX_REBOOT_CMD_KEXEC`` has been disabled.
> > +This value defaults to 0 (false: ``LINUX_REBOOT_CMD_KEXEC`` enabled),
> > +but can be set to 1 (true: ``LINUX_REBOOT_CMD_KEXEC`` disabled).
> > +Once true, kexec can no longer be used for reboot and the toggle
> > +cannot be set back to false.
> > +This toggle does not affect the use of kexec during a crash.
> > +
> > +
> >  kptr_restrict
> >  =============
> >
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index 41a686996aaa..15c3fad8918b 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -407,6 +407,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
> >  extern struct kimage *kexec_image;
> >  extern struct kimage *kexec_crash_image;
> >  extern int kexec_load_disabled;
> > +extern int kexec_reboot_disabled;
> >
> >  #ifndef kexec_flush_icache_page
> >  #define kexec_flush_icache_page(page)
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index cb8e6e6f983c..43063f803d81 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -196,6 +196,10 @@ static inline int kexec_load_check(unsigned long nr_segments,
> >       if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> >               return -EPERM;
> >
> > +     /* Check if the system admin has disabled kexec reboot. */
> > +     if (!(flags & KEXEC_ON_CRASH) && kexec_reboot_disabled)
> > +             return -EPERM;
>
> ... Allowing to load a crashkernel doesn't make sense in my opinion. If
> an attacker is capable of creating a malicious kernel, planting it on
> the victims system and then find a way to boot it via kexec this
> attacker also knows how to load the malicious kernel as crashkernel and
> trigger a panic. So you haven't really gained anything. That's why I
> would simply drop this hunk (and the corresponding one from
> kexec_file_load) and let users who worry about this use a combination of
> kexec_load_disabled and kexec_reboot_disabled.

If for whatever reason your sysadmin configured kexec_reboot_disabed
it can be nice that when a user try to load it they get a warning.
It is easier to debug than waiting two steps later when they run kexec -e....

That is why I added it. But i am also ok removing it

>
> Thanks
> Philipp
>
> > +
> >       /* Permit LSMs and IMA to fail the kexec */
> >       result = security_kernel_load_data(LOADING_KEXEC_IMAGE, false);
> >       if (result < 0)
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index ca2743f9c634..fe82e2525705 100644
> > --- a/kernel/kexec_core.c
> > +++ b/kernel/kexec_core.c
> > @@ -929,6 +929,7 @@ int kimage_load_segment(struct kimage *image,
> >  struct kimage *kexec_image;
> >  struct kimage *kexec_crash_image;
> >  int kexec_load_disabled;
> > +int kexec_reboot_disabled;
> >  #ifdef CONFIG_SYSCTL
> >  static struct ctl_table kexec_core_sysctls[] = {
> >       {
> > @@ -941,6 +942,16 @@ static struct ctl_table kexec_core_sysctls[] = {
> >               .extra1         = SYSCTL_ONE,
> >               .extra2         = SYSCTL_ONE,
> >       },
> > +     {
> > +             .procname       = "kexec_reboot_disabled",
> > +             .data           = &kexec_reboot_disabled,
> > +             .maxlen         = sizeof(int),
> > +             .mode           = 0644,
> > +             /* only handle a transition from default "0" to "1" */
> > +             .proc_handler   = proc_dointvec_minmax,
> > +             .extra1         = SYSCTL_ONE,
> > +             .extra2         = SYSCTL_ONE,
> > +     },
> >       { }
> >  };
> >
> > @@ -1138,7 +1149,7 @@ int kernel_kexec(void)
> >
> >       if (!kexec_trylock())
> >               return -EBUSY;
> > -     if (!kexec_image) {
> > +     if (!kexec_image || kexec_reboot_disabled) {
> >               error = -EINVAL;
> >               goto Unlock;
> >       }
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index 45637511e0de..583fba6de5cb 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -333,6 +333,11 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> >       if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> >               return -EPERM;
> >
> > +     /* Check if the system admin has disabled kexec reboot. */
> > +     if (!(flags & (KEXEC_FILE_ON_CRASH | KEXEC_FILE_UNLOAD))
> > +         && kexec_reboot_disabled)
> > +             return -EPERM;
> > +
> >       /* Make sure we have a legal set of flags */
> >       if (flags != (flags & KEXEC_FILE_FLAGS))
> >               return -EINVAL;
> >
>


-- 
Ricardo Ribalda