On 2024-10-08 at 19:56+0000 Sean Christopherson wrote: > Another (slightly crazy) approach would be use protection keys to provide the > security properties that you want, while giving KVM (and userspace) a quick-and-easy > override to access guest memory. > > 1. mmap() guest_memfd into userpace with RW protections > 2. Configure PKRU to make guest_memfd memory inaccessible by default > 3. Swizzle PKRU on-demand when intentionally accessing guest memory > > It's essentially the same idea as SMAP+STAC/CLAC, just applied to guest memory > instead of to usersepace memory. > > The benefit of the PKRU approach is that there are no PTE modifications, and thus > no TLB flushes, and only the CPU that is access guest memory gains temporary > access. The big downside is that it would be limited to modern hardware, but > that might be acceptable, especially if it simplifies KVM's implementation. Yeah this might be worth it if it simplifies significantly. Jenkins et al. showed MPK worked for stopping in-process Spectre V1 [1]. While future hardware bugs are always possible, the host kernel would still offer better protection overall since discovery of additional Spectre approaches and gadgets in the kernel is more likely (I think it's a bigger surface area than hardware-specific MPK transient execution issues). Patrick, we talked about this a couple weeks ago and ended up focusing on within-userspace protection, but I see keys can also be used to stop kernel access like Andrew's project he mentioned during Dave's MPK session at LPC [2]. Andrew, could you share that here? It's not clear to me how reliably the kernel prevents its own access to such pages. I see a few papers that warrant more investigation: "we found multiple interfaces that Linux, by design, provides for accessing process memory that ignore PKU domains on a page." [3] "Though Connor et al. demonstrate that existing MPK protections can be bypassed by using the kernel as a confused deputy, compelling recent work indicates that MPK operations can be made secure." [4] Dave and others, if you're aware of resources clarifying how strong the boundaries are, that would be helpful. Derek [1] https://www.cs.dartmouth.edu/~sws/pubs/jas2020.pdf [2] https://www.youtube.com/watch?v=gEUeMfrNH94&t=1028s [3] https://www.usenix.org/system/files/sec20-connor.pdf [4] https://ics.uci.edu/~dabrowsa/kirth-eurosys22-pkru.pdf
