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

On 2021-02-17 15:45, Marc Zyngier wrote:

On 2021-02-17 09:42, Will Deacon wrote:

quoted

[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:

quoted

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)

  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.

+1 on that. Even if, as I suspect, this is targeting some unspecified
hypervisor that is not KVM, the first course of action should be for
this to be implemented in the kernel's own hypervisor first so that
anyone can review understand what is at play.

Thanks,

         M.

Thank you for your comments. The key value add of the feature is a third
party independent entity keeping a watch on crucial kernel assets, such 
that
in case the kernel itself is compromised, still, the protection can 
remain intact.
Can this be achieved if the implementation is done in KVM? I've limited 
knowledge
of KVM currently, can surely look into more details for a better 
understanding.

Agree that the mechanism for triaging what constitutes valid access 
needs a clear
approach. We will discuss your suggestions internally if we can use them 
to improve
the overall feature design and share updated patches. Thank you!