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

[Paul Moore]

On Tue, Feb 16, 2021 at 5:19 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>
> ---
> 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(-)

As long as we are only talking about moving the selinux_state struct
itself and none of the pointers inside I think we should be okay (the
access decision cache pointed to by selinux_state->avc could change
frequently).  Have you done any performance measurements of this
change?  Assuming they are not terrible, I have no objections to this
patch from a SELinux perspective.

-- 
paul moore
www.paul-moore.com