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

On Tue, Feb 16, 2021 at 5:19 AM Preeti Nagar [off-list ref] 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 <redacted>
---
The RFC patch reviewed available at:
https://lore.kernel.org/linux-security-module/1610099389-28329-1-git-send-email-pnagar@codeaurora.org/ (local)
---
 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