Hi Ricardo, On Thu, 24 Nov 2022 23:32:34 +0100 Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote: > Hi Philipp > > > On Thu, 24 Nov 2022 at 16:01, Philipp Rudo <prudo@xxxxxxxxxx> wrote: > > > > On Thu, 24 Nov 2022 13:52:58 +0100 > > Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote: > > > > > On Thu, 24 Nov 2022 at 12:40, Philipp Rudo <prudo@xxxxxxxxxx> wrote: > > > > > > > > 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? > > > > > > 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? Then there are options like OnFailure= or FailureAction= the sysadmin can use do what ever he seems appropriate. > Or worse, your distro does not use systemd !!! In that case there are other ways to achieve the same. The two options are only examples. Why can't this be used as a replacement? > > 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. Well the same holds for kexec. So the only difference is triggering the panic where I'm still not convinced it's a huge obstacle for someone who is able to pull off all the steps before for such an attack. To be honest I don't think we make a progress here at the moment. I would like to hear from others what they think about this. Thanks Philipp > > > > > > 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@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; > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >