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    SubjectRe: [PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled
    Hi Philipp


    On Thu, 24 Nov 2022 at 16:01, Philipp Rudo <prudo@redhat.com> wrote:
    >
    > On Thu, 24 Nov 2022 13:52:58 +0100
    > Ricardo Ribalda <ribalda@chromium.org> wrote:
    >
    > > On Thu, 24 Nov 2022 at 12:40, Philipp Rudo <prudo@redhat.com> wrote:
    > > >
    > > > Hi Ricardo,
    > > >
    > > > On Wed, 23 Nov 2022 09:58:08 +0100
    > > > Ricardo Ribalda <ribalda@chromium.org> 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?
    > >
    > > Let's say that you have a script that does something like this
    > >
    > >
    > > kexec -p dump_kernel
    > > echo 1 > /proc/sys/kernel/kexec_load_disabled
    > >
    > > If an attacker can DDos the system and make that script crash... then
    > > kexec is still accessible
    > >
    > > On the other hand, if you load the kernel with the commandline
    > >
    > > sysctl.kernel.kexec_load_disabled=1
    > ^^^^
    > reboot?

    yes :) thanks!

    > Otherwise you shouldn't be able to load the crash kernel at all.
    >
    > > Then even if the script crashes, the only way to abuse kexec is by
    > > panicing the running kernel....
    >
    > True. But when an attacker can DDos the system the final workload is
    > already running. So wouldn't it be enough to make sure that the script
    > above has finished before starting you workload. E.g. by setting an
    > appropriate Before=/After= in the systemd.unit?

    What if the kexec binary crashes and the unit will never succeed?

    Or worse, your distro does not use systemd !!!

    >
    > Furthermore, I don't think that restricting kexec reboot alone is
    > sufficient when the attacker can still control the crash kernel. At
    > least my assumption is that triggering a panic instead of just
    > rebooting is just a mild inconvenience for somebody who is able to pull
    > off an attack like that.

    The attacker does not control the crash kernel completely. loadpin is
    still in place.
    Yes, they can downgrade the whole system to a vulnerable kernel image.
    But the choices are limited :)

    With physical access to the device panicing a kernel is easily doable
    (but not trivial). But remotely, it is more challenging.

    >
    > > Would it make you more comfortable if I model this as a kernel config
    > > instead of a runtime option?
    >
    > No, I think the implementation is fine. I'm currently only struggling
    > to understand what problem kexec_reboot_disabled solves that cannot be
    > solved by kexec_load_disabled.
    >
    > > Thanks!
    >
    > Happy to help.
    >
    > Thanks
    > Philipp
    >
    > >
    > >
    > > >
    > > > Thanks
    > > > Philipp
    > > >
    > > > >
    > > > > On Mon, 21 Nov 2022 at 15:10, Philipp Rudo <prudo@redhat.com> wrote:
    > > > > >
    > > > > > Hi Ricardo,
    > > > > >
    > > > > > On Thu, 17 Nov 2022 16:15:07 +0100
    > > > > > Ricardo Ribalda <ribalda@chromium.org> wrote:
    > > > > >
    > > > > > > Hi Philipp
    > > > > > >
    > > > > > > Thanks for your review!
    > > > > >
    > > > > > happy to help.
    > > > > >
    > > > > > >
    > > > > > > On Thu, 17 Nov 2022 at 16:07, Philipp Rudo <prudo@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@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@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....
    > > > > >
    > > > > > 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;
    > > > > > > > >
    > > > > > > >
    > > > > > >
    > > > > > >
    > > > > >
    > > > >
    > > > >
    > > >
    > >
    > >
    >


    --
    Ricardo Ribalda

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