Re: [RFC PATCH v2] selinux: security: Move selinux_state to a separate page

[Casey Schaufler <casey@schaufler-ca.com>]

On 1/8/2021 1:49 AM, Preeti Nagar wrote:
> The changes introduce a new security feature, RunTime Integrity Check
> (RTIC), designed to protect Linux Kernel at runtime. The motivation
> behind these changes is:
> 1. The system protection offered by SE for Android relies on the
> assumption of kernel integrity. If the kernel itself is compromised (by
> a perhaps as yet unknown future vulnerability), SE for Android security
> mechanisms could potentially be disabled and rendered ineffective.
> 2. Qualcomm Snapdragon devices use Secure Boot, which adds cryptographic
> checks to each stage of the boot-up process, to assert the authenticity
> of all secure software images that the device executes.  However, due to
> various vulnerabilities in SW modules, the integrity of the system can be
> compromised at any time after device boot-up, leading to un-authorized
> SW executing.

It would be helpful if you characterized the "various vulnerabilities"
rather than simply asserting their existence. This would allow the reviewer
to determine if the proposed patch addresses the issue.

>
> The feature's idea is to move some sensitive kernel structures to a
> separate page and monitor further any unauthorized changes to these,
> from higher Exception Levels using stage 2 MMU. Moving these to a
> different page will help avoid getting page faults from un-related data.

I've always been a little slow when it comes to understanding the
details of advanced memory management facilities. That's part of
why I work in access control. Could you expand this a bit, so that
someone who doesn't already know how your stage 2 MMU works might
be able to evaluate what you're doing here.

> Using this mechanism, some sensitive variables of the kernel which are
> initialized after init or are updated rarely can also be protected from
> simple overwrites and attacks trying to modify these.

How would this interact with or complement __read_mostly?

>
> Currently, the change moves selinux_state structure to a separate page. In
> future we plan to move more security-related kernel assets to this page to
> enhance protection.

What's special about selinux_state? What about the SELinux policy?
How would I, as maintainer of the Smack security module, know if
some Smack data should be treated the same way? 

>
> We want to seek your suggestions and comments on the idea and the changes
> in the patch.
>
> Signed-off-by: Preeti Nagar <pnagar@codeaurora.org>
> ---
>  include/asm-generic/vmlinux.lds.h | 10 ++++++++++
>  include/linux/init.h              |  4 ++++
>  security/Kconfig                  | 10 ++++++++++
>  security/selinux/hooks.c          |  4 ++++
>  4 files changed, 28 insertions(+)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b2b3d81..158dbc2 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -770,6 +770,15 @@
>  		*(.scommon)						\
>  	}
>  
> +#ifdef CONFIG_SECURITY_RTIC
> +#define RTIC_BSS							\
> +	. = ALIGN(PAGE_SIZE);						\
> +	KEEP(*(.bss.rtic))						\
> +	. = ALIGN(PAGE_SIZE);
> +#else
> +#define RTIC_BSS
> +#endif
> +
>  /*
>   * Allow archectures to redefine BSS_FIRST_SECTIONS to add extra
>   * sections to the front of bss.
> @@ -782,6 +791,7 @@
>  	. = ALIGN(bss_align);						\
>  	.bss : AT(ADDR(.bss) - LOAD_OFFSET) {				\
>  		BSS_FIRST_SECTIONS					\
> +		RTIC_BSS						\
>  		. = ALIGN(PAGE_SIZE);					\
>  		*(.bss..page_aligned)					\
>  		. = ALIGN(PAGE_SIZE);					\
> diff --git a/include/linux/init.h b/include/linux/init.h
> index 7b53cb3..617adcf 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -300,6 +300,10 @@ void __init parse_early_options(char *cmdline);
>  /* Data marked not to be saved by software suspend */
>  #define __nosavedata __section(".data..nosave")
>  
> +#ifdef CONFIG_SECURITY_RTIC
> +#define __rticdata  __section(".bss.rtic")
> +#endif
> +
>  #ifdef MODULE
>  #define __exit_p(x) x
>  #else
> diff --git a/security/Kconfig b/security/Kconfig
> index 7561f6f..66b61b9 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -291,5 +291,15 @@ config LSM
>  
>  source "security/Kconfig.hardening"
>  
> +config SECURITY_RTIC
> +        bool "RunTime Integrity Check feature"

Shouldn't this depend on the architecture(s) supporting the
feature?

> +        help
> +	  RTIC(RunTime Integrity Check) feature is to protect Linux kernel
> +	  at runtime. This relocates some of the security sensitive kernel
> +	  structures to a separate page aligned special section.
> +
> +	  This is to enable monitoring and protection of these kernel assets
> +	  from a higher exception level(EL) against any unauthorized changes.

"if you are unsure ..."

> +
>  endmenu
>  
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 6b1826f..7add17c 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -104,7 +104,11 @@
>  #include "audit.h"
>  #include "avc_ss.h"
>  
> +#ifdef CONFIG_SECURITY_RTIC
> +struct selinux_state selinux_state __rticdata;
> +#else
>  struct selinux_state selinux_state;
> +#endif

Shouldn't the __rticdata tag be applied always, and its
definition take care of the cases where it doesn't do anything?

>  
>  /* SECMARK reference count */
>  static atomic_t selinux_secmark_refcount = ATOMIC_INIT(0);