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

[pnagar@codeaurora.org]

On 2021-02-17 02:02, Miguel Ojeda wrote:
> On Tue, Feb 16, 2021 at 11:22 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.
> 
> 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