[PATCH v3] kexec: add sysctl to disable kexec_load

[Vivek Goyal]

On Wed, Dec 11, 2013 at 03:54:27PM -0800, Kees Cook wrote:
> For general-purpose (i.e. distro) kernel builds it makes sense to build with
> CONFIG_KEXEC to allow end users to choose what kind of things they want to do
> with kexec. However, in the face of trying to lock down a system with such
> a kernel, there needs to be a way to disable kexec_load (much like module
> loading can be disabled). Without this, it is too easy for the root user to
> modify kernel memory even when CONFIG_STRICT_DEVMEM and modules_disabled are
> set. With this change, it is still possible to load an image for use later,
> then disable kexec_load so the image (or lack of image) can't be altered.
> 

Hi Kees,

I am still not able to wrap my head around that how it will be used in 
practice.

So you seem to be planning that user space will load a kdump kernel early
and then disable further load of any kexec/kdump images.

We are doing all this to protect against root loading a image we don't
want and it is root who will load an image to begin with. Your argument
is that if root changes the init scripts, then those changes will take
affect only during next reboot and will be detected.

I don't understand that how would you enforce it. What if root changes it
and just waits for next scheduled maintenance reboot. So this does not
sound very convincing to me. If there was a way to disable kexec
completely using command line, then I could understand that in some
cases hypervisor will control how virtual machines are launched with
hypervisor determined command line paramaters and hypervisor could
enforce that kexec/kdump are disabled and root in virtual machine can't
do anything about it.

But we seem to be first trusting root to disable kexec/kdump and then 
little later not trusting same root and expecting that if that root
changes something it will take affect only after a reboot and we will
notice it. I am not convinced about the *noticing a reboot* part.

So while patch is simple and seems to be making sense, what's the
exact use case and how it will be enforced is still unclear to me.

Thanks
Vivek


> Signed-off-by: Kees Cook <keescook at chromium.org>
> Acked-by: Rik van Riel <riel at redhat.com>
> ---
> v3:
>  - renamed and clarified to kexec_load_disabled; Eric W. Biederman
> v2:
>  - updated sysctl documentation; akpm
> ---
>  Documentation/sysctl/kernel.txt |   15 ++++++++++++++-
>  include/linux/kexec.h           |    1 +
>  kernel/kexec.c                  |    3 ++-
>  kernel/sysctl.c                 |   13 +++++++++++++
>  4 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 26b7ee491df8..3e1846427eda 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -33,6 +33,7 @@ show up in /proc/sys/kernel:
>  - domainname
>  - hostname
>  - hotplug
> +- kexec_load_disabled
>  - kptr_restrict
>  - kstack_depth_to_print       [ X86 only ]
>  - l2cr                        [ PPC only ]
> @@ -287,6 +288,18 @@ Default value is "/sbin/hotplug".
>  
>  ==============================================================
>  
> +kexec_load_disabled:
> +
> +A toggle indicating if the kexec_load syscall has been disabled. This
> +value defaults to 0 (false: kexec_load enabled), but can be set to 1
> +(true: kexec_load disabled). Once true, kexec can no longer be used, and
> +the toggle cannot be set back to false. This allows a kexec image to be
> +loaded before disabling the syscall, allowing a system to set up (and
> +later use) an image without it being altered. Generally used together
> +with the "modules_disabled" sysctl.
> +
> +==============================================================
> +
>  kptr_restrict:
>  
>  This toggle indicates whether restrictions are placed on
> @@ -331,7 +344,7 @@ A toggle value indicating if modules are allowed to be loaded
>  in an otherwise modular kernel.  This toggle defaults to off
>  (0), but can be set true (1).  Once true, modules can be
>  neither loaded nor unloaded, and the toggle cannot be set back
> -to false.
> +to false.  Generally used with the "kexec_load_disabled" toggle.
>  
>  ==============================================================
>  
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index d78d28a733b1..a3e842f6867e 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -170,6 +170,7 @@ unsigned long paddr_vmcoreinfo_note(void);
>  
>  extern struct kimage *kexec_image;
>  extern struct kimage *kexec_crash_image;
> +extern int kexec_load_disabled;
>  
>  #ifndef kexec_flush_icache_page
>  #define kexec_flush_icache_page(page)
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 490afc03627e..9405ae68feb4 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -929,6 +929,7 @@ static int kimage_load_segment(struct kimage *image,
>   */
>  struct kimage *kexec_image;
>  struct kimage *kexec_crash_image;
> +int kexec_load_disabled;
>  
>  static DEFINE_MUTEX(kexec_mutex);
>  
> @@ -939,7 +940,7 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
>  	int result;
>  
>  	/* We only trust the superuser with rebooting the system. */
> -	if (!capable(CAP_SYS_BOOT))
> +	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
>  		return -EPERM;
>  
>  	/*
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 34a604726d0b..ea4bb8152a34 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -62,6 +62,7 @@
>  #include <linux/capability.h>
>  #include <linux/binfmts.h>
>  #include <linux/sched/sysctl.h>
> +#include <linux/kexec.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/processor.h>
> @@ -614,6 +615,18 @@ static struct ctl_table kern_table[] = {
>  		.proc_handler	= proc_dointvec,
>  	},
>  #endif
> +#ifdef CONFIG_KEXEC
> +	{
> +		.procname	= "kexec_load_disabled",
> +		.data		= &kexec_load_disabled,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		/* only handle a transition from default "0" to "1" */
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &one,
> +		.extra2		= &one,
> +	},
> +#endif
>  #ifdef CONFIG_MODULES
>  	{
>  		.procname	= "modprobe",
> -- 
> 1.7.9.5
> 
> 
> -- 
> Kees Cook
> Chrome OS Security