Re: [PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled

[Philipp Rudo]

Hi Ricardo,

On Wed, 23 Nov 2022 09:58:08 +0100
Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:

> Hi Philipp
> 
> Thanks for your review.
> 
> My scenario is a trusted system, where even if you are root, your
> access to the system is very limited.
> 
> Let's assume LOADPIN and verity are enabled.

My point is that on such systems I expect that a sysadmin also wants to
control the crash kernel including its initramfs (which also has to be part
of the signed kernel?). But if that's the case a sysadmin can simply arm
kdump early during boot and then toggle kexec_load_disabled. With that
LINUX_REBOOT_CMD_KEXEC also gets disabled as no kexec kernel can be loaded
while kdump works. Thus there is no need to add the new interface. Or am
I missing anything?

Thanks
Philipp

> 
> On Mon, 21 Nov 2022 at 15:10, Philipp Rudo <prudo@xxxxxxxxxx> wrote:
> >
> > Hi Ricardo,
> >
> > On Thu, 17 Nov 2022 16:15:07 +0100
> > Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:
> >  
> > > Hi Philipp
> > >
> > > Thanks for your review!  
> >
> > happy to help.
> >  
> > >
> > > On Thu, 17 Nov 2022 at 16:07, Philipp Rudo <prudo@xxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > >
> > > > > 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....  
> >
> > I'm having second thoughts about this patch. My main problem is that I
> > don't see a real use case where kexec_reboot_disabled is advantageous
> > over kexec_load_disabled. The point is that disabling
> > LINUX_REBOOT_CMD_KEXEC is almost identical to toggling kexec_load_disabled without
> > a loaded kernel (when you don't have a kernel loaded you cannot reboot
> > into it). With this the main use case of kexec_reboot_disabled is
> > already covered by kexec_load_disabled.  
> 
> >
> > However, there are two differences
> >
> > 1) with kexec_reboot_disable you can still (re-)load a crash kernel
> > e.g. to update the initramfs after a config change. But as discussed in
> > my first mail this comes on the cost that an attacker could still load a
> > malicious crash kernel and then 'panic into it'.  
> 
> That crash kernel must be already in the signed malicious kernel.
> which reduces the chances of attack.
> Plus an attacker must be able to panic the current kernel at will,
> instead of just call reset.
> 
> >
> > 2) kexec_load_disabled also prevents unloading of a loaded kernel. So
> > once loaded kexec_load_disabled cannot prevent the reboot into this
> > kernel.
> >
> >
> > For 1) I doubt that this is desired at all. My expectation is that on
> > systems where a sysadmin restricts a user to reboot via kexec the
> > sysadmin also wants to prevent the user to load an arbitrary crash
> > kernel. Especially as this still keeps the loophole open you are trying
> > to close.
> >
> > So only 2) is left as real benefit. But that is an extremely specific
> > scenario. How often does this scenario happen in real life? What
> > problem does kexec_reboot_disable solve different implementation
> > (also in userspace) cannot?
> >
> > Sorry about being this pedantic but you want to introduce some new uapi
> > which will be hard if not impossible to change once introduced. That's
> > why I want to be a 100% sure it is really needed.  
> 
> No worries. Completely understand :). Thanks for taking this seriously..
> 
> 
> Best regards!
> >
> > Thanks
> > Philipp
> >
> >  
> > > 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;
> > > > >  
> > > >  
> > >
> > >  
> >  
> 
>