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

From: Brijesh Singh
Date: Mon May 03 2021 - 13:31:43 EST

On 5/3/21 12:19 PM, Brijesh Singh wrote:
> On 5/3/21 11:15 AM, Dave Hansen wrote:
>> 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

One small correction, I was meaning to say VIRTIO but typed VFIO. Sorry
for the confusion.

>> an address such that the host kernel would blindly try to write private
>> guest memory?
> Not blindly. But a guest could trick a VMM (qemu) to ask the host driver
> to access a GPA which is guest private page (Its a hypothetical case, so
> its possible that I may missing something). Let's see with an example:
> - A guest provides a GPA to VMM to write to (e.g DMA operation).
> - VMM translates the GPA->HVA and calls down to host kernel with the HVA.
> - The host kernel may pin the HVA to get the PFN for it and then kmap().
> Write to the mapped PFN will cause an RMP fault if the guest provided
> GPA was not a marked shared in the RMP table. In an ideal world, a guest
> should *never* do this but what if it does ?
>> 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?
> The GPA->HVA is typically done by the VMM, and HVA->HPA is done by the
> host drivers. So, only time we could verify is after the HVA->HPA. One
> of my patch provides a snp_lookup_page_in_rmptable() helper that can be
> used to query the page state in the RMP table. This means the all the
> host backend drivers need to enlightened to always read the RMP table
> before making a write access to guest provided GPA. A good guest should
> *never* be using a private page for the DMA operation and if it does
> then the fault handler introduced in this patch can avoid the host crash
> and eliminate the need to enlightened the drivers to check for the
> permission before the access.
> I felt it is good idea to have some kind of recovery specially when a
> malicious guest could lead us into this path.
>>> 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.
> Yes, fine with me.
>>>> 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?
> I am trying is a cover a malicious guest cases. A good guest should
> follow the GHCB spec and change the page state before the access.
>>> 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.