[PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled

[Philipp Rudo]

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.

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;
>