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

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On 2021-02-17 02:02, Miguel Ojeda wrote:
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.

Yes, that will be helpful, will put it in Documentation/security in the
next update. Thank you!

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.

Thanks :)

Cheers,
Miguel



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