Hi Dave, On 5/3/21 9:44 AM, Dave Hansen wrote: > On 4/30/21 5:37 AM, Brijesh Singh wrote: >> When SEV-SNP is enabled globally, a write from the host goes through the >> RMP check. When the host writes to pages, hardware checks the following >> conditions at the end of page walk: >> >> 1. Assigned bit in the RMP table is zero (i.e page is shared). >> 2. If the page table entry that gives the sPA indicates that the target >> page size is a large page, then all RMP entries for the 4KB >> constituting pages of the target must have the assigned bit 0. >> 3. Immutable bit in the RMP table is not zero. >> >> The hardware will raise page fault if one of the above conditions is not >> met. A host should not encounter the RMP fault in normal execution, but >> a malicious guest could trick the hypervisor into it. e.g., a guest does >> not make the GHCB page shared, on #VMGEXIT, the hypervisor will attempt >> to write to GHCB page. > Is that the only case which is left? If so, why don't you simply split > the direct map for GHCB pages before giving them to the guest? Or, map > them with vmap() so that the mapping is always 4k? GHCB was just an example. Another example is a vfio driver accessing the shared page. If those pages are not marked shared then kernel access will cause an RMP fault. Ideally we should not be running into this situation, but if we do, then I am trying to see how best we can avoid the host crashes. Another reason for having this is to catch the hypervisor bug, during the SNP guest create, the KVM allocates few backing pages and sets the assigned bit for it (the examples are VMSA, and firmware context page). If hypervisor accidentally free's these pages without clearing the assigned bit in the RMP table then it will result in RMP fault and thus a kernel crash. > > Or, worst case, you could use exception tables and something like > copy_to_user() to write to the GHCB. That way, the thread doing the > write can safely recover from the fault without the instruction actually > ever finishing execution. > > BTW, I went looking through the spec. I didn't see anything about the > guest being able to write the "Assigned" RMP bit. Did I miss that? > Which of the above three conditions is triggered by the guest failing to > make the GHCB page shared? The GHCB spec section "Page State Change" provides an interface for the guest to request the page state change. During bootup, the guest uses the Page State Change VMGEXIT to request hypervisor to make the page shared. The hypervisor uses the RMPUPDATE instruction to write to "assigned" bit in the RMP table. On VMGEXIT, the very first thing which vmgexit handler does is to map the GHCB page for the access and then later using the copy_to_user() to sync the GHCB updates from hypervisor to guest. The copy_to_user() will cause a RMP fault if the GHCB is not mapped shared. As I explained above, GHCB page was just an example, vfio or other may also get into this situation. -Brijesh
