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

[Dave Hansen]

On 5/3/21 8:37 AM, Brijesh Singh wrote:
> 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.

I'm confused.  Are you suggesting that the VFIO driver could be passed
an address such that the host kernel would blindly try to write private
guest memory?

The host kernel *knows* which memory is guest private and what is
shared.  It had to set it up in the first place.  It can also consult
the RMP at any time if it somehow forgot.

So, this scenario seems to be that the host got a guest physical address
(gpa) from the guest, it did a gpa->hpa->hva conversion and then wrote
the page all without bothering to consult the RMP.  Shouldn't the the
gpa->hpa conversion point offer a perfect place to determine if the page
is shared or private?

> 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.

I think I'd be just fine with a BUG_ON() in those cases instead of an
attempt to paper over the issue.  Kernel crashes are fine in the case of
kernel bugs.

>> 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.

Right...  So the *HOST* is in control.  Why should the host ever be
surprised by a page transitioning from shared to private?

> 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.

Causing an RMP fault is fine.  The problem is shoving a whole bunch of
*recovery* code in the kernel when recovery isn't necessary.  Just look
for the -EFAULT from copy_to_user() and move on with life.