Re: [PATCH] RTIC: selinux: ARM64: Move selinux_state to a separate page

From: Nick Desaulniers
Date: Tue Feb 16 2021 - 13:10:45 EST

On Tue, Feb 16, 2021 at 2:19 AM Preeti Nagar <pnagar@xxxxxxxxxxxxxx> 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 Security Enhancements(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.
>
> 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.
> The mechanism we have been working on removes the write permissions for
> HLOS in the stage 2 page tables for the regions to be monitored, such
> that any modification attempts to these will lead to faults being
> generated and handled by handlers. If the protected assets are moved to
> a separate page, faults will be generated corresponding to change attempts
> to these assets only. If not moved to a separate page, write attempts to
> un-related data present on the monitored pages will also be generated.
>
> Using this feature, 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.
>
> Currently, the change moves selinux_state structure to a separate page.
> The page is 2MB aligned not 4K to avoid TLB related performance impact as,
> for some CPU core designs, the TLB does not cache 4K stage 2 (IPA to PA)
> mappings if the IPA comes from a stage 1 mapping. In future, we plan to
> move more security-related kernel assets to this page to enhance
> protection.
>
> Signed-off-by: Preeti Nagar <pnagar@xxxxxxxxxxxxxx>

This addresses my feedback from the RFC regarding the section symbols.
No comment on whether there is a better approach, or the 2MB vs page
alignment, but perhaps other folks cc'ed can please take a look.

Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

> ---
> The RFC patch reviewed available at:
> https://lore.kernel.org/linux-security-module/1610099389-28329-1-git-send-email-pnagar@xxxxxxxxxxxxxx/
> ---
> include/asm-generic/vmlinux.lds.h | 10 ++++++++++
> include/linux/init.h | 6 ++++++
> security/Kconfig | 11 +++++++++++
> security/selinux/hooks.c | 2 +-
> 4 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b97c628..d1a5434 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(SZ_2M); \
> + KEEP(*(.bss.rtic)) \
> + . = ALIGN(SZ_2M);
> +#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 e668832..e6d452a 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -300,6 +300,12 @@ 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")
> +#else
> +#define __rticdata
> +#endif
> +
> #ifdef MODULE
> #define __exit_p(x) x
> #else
> diff --git a/security/Kconfig b/security/Kconfig
> index 7561f6f..1af913a 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -291,5 +291,16 @@ config LSM
>
> source "security/Kconfig.hardening"
>
> +config SECURITY_RTIC
> + bool "RunTime Integrity Check feature"
> + depends on ARM64
> + 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 RTIC specific page.
> +
> + This is to enable monitoring and protection of these kernel assets
> + from a higher exception level(EL) against any unauthorized changes.
> +
> endmenu
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 644b17e..59d7eee 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -104,7 +104,7 @@
> #include "audit.h"
> #include "avc_ss.h"
>
> -struct selinux_state selinux_state;
> +struct selinux_state selinux_state __rticdata;
>
> /* SECMARK reference count */
> static atomic_t selinux_secmark_refcount = ATOMIC_INIT(0);
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
> of Code Aurora Forum, hosted by The Linux Foundation
>


--
Thanks,
~Nick Desaulniers