Messages in this thread | | | Date | Mon, 22 Feb 2021 12:44:29 +0530 | From | pnagar@codeauro ... | Subject | Re: [PATCH] RTIC: selinux: ARM64: Move selinux_state to a separate page |
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On 2021-02-17 15:23, Ard Biesheuvel wrote: > On Wed, 17 Feb 2021 at 10:42, Will Deacon <will@kernel.org> 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) >> >> 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. >> > > Agree here. Making an arbitrary set of data structures r/o behind the > OS's back doesn't seem like an easy thing to maintain or reason about, > especially if this r/o-ness is only enforced on a tiny subset of > devices. If something needs to be writable only at boot, we have > __ro_after_init, and having hypervisor assisted enforcement of /that/ > might be a worthwhile thing to consider, including perhaps ways to do > controlled patching of this region at runtime. >
Thank you for the suggestions. We will look into the possibility of protection of __ro_after_init and controlled updates to these. I understand, if this can be made generic as Will also suggested, it might be more useful and easy to scale and maintain.
>> 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
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