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

[peter enderborg]

On 2/17/21 10:42 AM, Will Deacon wrote:
> [Please include arm64 and kvm folks for threads involving the stage-2 MMU]
>
> On Tue, Feb 16, 2021 at 03:47:52PM +0530, Preeti Nagar 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.
> Although I really like the idea of using stage-2 to protect the kernel, I
> think the approach you outline here is deeply flawed. Identifying "sensitive
> variables" of the kernel to protect is subjective and doesn't scale.
> Furthermore, the triaging of what constitues a valid access is notably
> absent from your description and is assumedly implemented in an opaque blob
> at EL2.
>
> I think a better approach would be along the lines of:
>
>   1. Introduce the protection at stage-1 (like we already have for mapping
>      e.g. the kernel text R/O)

Will that really solve the problem? There is a lot of caches that are used
to resolve policy data in selinux, and this caches will not be protected.
If you can manipulate kernel data you can do cache poisoning.


>   2. Implement the handlers in the kernel, so the heuristics are clear.
>
>   3. Extend this to involve KVM, so that the host can manage its own
>      stage-2 to firm-up the stage-1 protections.
>
> I also think we should avoid tying this to specific data structures.
> Rather, we should introduce a mechanism to make arbitrary data read-only.
>
> I've CC'd Ard and Marc, as I think they've both been thinking about this
> sort of thing recently as well.
>
> Will