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

Philipp Rudo
2022-11-17T15:08:38Z

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.

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;
>