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
