[PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled

[Baoquan He]

On 11/14/22 at 02:18pm, Ricardo Ribalda wrote:
> Create a new toogle that disables LINUX_REBOOT_CMD_KEXEC, reducing the
               ~ toggle
> attack surface to a system.
> 
> Without this toogle, an attacker can only reboot into a different kernel
  ~~s/without/with/
> if they can create a panic().

The log just tells what it's doing, but miss the most important why this
has to be needed, the motivation.

I roughly read the talking between you and Philipp, wondering why do you
believe panicked kernel, if you worry about the untrusted kernel kexec
rebooted into. People can change scripts in initramfs, e.g drop into
emergency shell and switch into rootfs, there are a lot of things people
can do in there too.

> 
> 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;
> +
>  	/* 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;
> 
> -- 
> b4 0.11.0-dev-d93f8
>