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

[Miguel Ojeda]

On Tue, Feb 16, 2021 at 11:22 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.

Part of this commit message should likely be added as a new file under
Documentation/ somewhere.

> 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.

Rewording suggestion:

         The RTIC (RunTime Integrity Check) feature protects the kernel
         at runtime by relocating some of its security-sensitive structures
         to a separate RTIC-specific page. This enables monitoring and
         and protecting them from a higher exception level against
         unauthorized changes.

Cheers,
Miguel