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

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Feb 16, 2021 at 2:19 AM Preeti Nagar <pnagar@codeaurora.org> 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@codeaurora.org>

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@google.com>

> ---
> The RFC patch reviewed available at:
> https://lore.kernel.org/linux-security-module/1610099389-28329-1-git-send-email-pnagar@codeaurora.org/
> ---
>  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