On 09/10/2024 4:51 am, Manwaring, Derek wrote:
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?
This was in reference to PKS specifically (so Sapphire Rapids and later), and also for Xen but the technique is general.
Allocate one supervisor key for the directmap (and other ranges wanting protecting), and configure MSR_PKS[key]=AD by default.
Protection Keys were identified as being safe as a defence against Meltdown. At the time, only PKRU existed, and PKS was expected to have been less overhead than KPTI on Skylake, which was even more frustrating for those of us who'd begged for a supervisor form at the time. What's done is done.
The changes needed in main code would be accessors for directmap pointers, because there needs to temporary AD-disable. This would take the form of 2x WRMSR, as opposed to a STAC/CLAC pair.
An area of concern is the overhead of the WRMSRs. MSR_PKS is defined as not-architecturally-serialising, but like STAC/CLAC probably comes with model-dependent dispatch-serialising properties to prevent memory accesses executing speculatively under the wrong protection key.
Also, for this strategy to be effective, you need to PKEY-tag all aliases of the memory.
~Andrew