Re: [PATCH Part2 RFC v2 10/37] x86/fault: Add support to handle the RMP fault for kernel address

From: Andy Lutomirski
Date: Mon May 03 2021 - 11:04:02 EST

On Mon, May 3, 2021 at 7:44 AM Dave Hansen <dave.hansen@xxxxxxxxx> 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?

If I read Brijesh's message right, this isn't about 4k. It's about
the guest violating host expectations about the page type.

I need to go and do a full read of all the relevant specs, but I think
there's an analogous situation in TDX: if the host touches guest
private memory, the TDX hardware will get extremely angry (more so
than AMD hardware). And, if I have understood this patch correctly,
it's fudging around the underlying bug by intentionally screwing up
the RMP contents to avoid a page fault. Assuming I've understood
everything correctly (a big if!), then I think this is backwards. The
host kernel should not ever access guest memory without a plan in
place to handle failure. We need real accessors, along the lines of
copy_from_guest() and copy_to_guest().