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

[Brijesh Singh]

On 5/3/21 12:40 PM, Andy Lutomirski wrote:
>
>> On May 3, 2021, at 10:19 AM, Brijesh Singh <brijesh.singh@xxxxxxx> 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
>>> 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.
> Can we arrange for the page walk plus kmap process to fail?

Sure, I will look into all the drivers which do a walk plus kmap to make
sure that they fail instead of going into the fault path. Should I drop
this patch or keep it just in the case we miss something?