On Wed, 17 Feb 2021 at 10:42, Will Deacon <will@xxxxxxxxxx> 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. > 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
