Re: [Xen-devel] SVM NPT mem_access and npfec.insn_fetch

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/03/2018 22:47, Razvan Cojocaru wrote:
> On 03/31/2018 12:14 AM, Andrew Cooper wrote:
>> On 30/03/2018 22:06, Razvan Cojocaru wrote:
>>> Hello,
>>>
>>> We've been trying to enable mem_access events for SVM, and we do receive
>>> some events with xen-access if we set the pages read-only. However, the
>>> exec xen-access tests fails, because it appears that npfec.insn_fetch is
>>> always 0 here (or at least we haven't seen it to be 1 in a lot of testing):
>>>
>>> index 569b124..d518c94 100644
>>> --- a/xen/arch/x86/hvm/hvm.c
>>> +++ b/xen/arch/x86/hvm/hvm.c
>>> @@ -1835,6 +1835,13 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
>>> unsigned long gla,
>>>              break;
>>>          }
>>>
>>> +        printk("pae: %d, nx: %d, gfn: 0x%lx access %d violation %d read %d"
>>> +               " write %d insn %d present %d glav %d kind %d \n",
>>> +               !!(curr->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PAE),
>>> +               !!(curr->arch.hvm_vcpu.guest_efer & EFER_NX), gfn,
>>> +               p2ma, violation, npfec.read_access, npfec.write_access,
>>> +               npfec.insn_fetch, npfec.present, npfec.gla_valid,
>>> npfec.kind);
>>> +
>>>          if ( violation )
>>>          {
>>>              /* Should #VE be emulated for this fault? */
>>>
>>> This patch does not require anything else, just start an HVM guest on an
>>> AMD host. It outputs things like:
>>>
>>> (XEN) pae: 1, nx: 1, gfn: 0xf0235 access 7 violation 0 read 1 write 1
>>> insn 0 present 1 glav 0 kind 2
>>> [...]
>>> What are we missing?
>> Look at p2m-pt.c and check whether NX is being properly set in the NPT
>> tables when you select XENMEM_access_rw ?  NPT pagetables have an
>> identical format and layout to regular PAE pagetables.
> We weren't setting it at all. Just stored the restrictions in a
> newly-added integer in struct page_info (suggestions for the best place
> to store them are welcome), and fed them back to whatever function was
> asking for them from there.
>
> Did a quick test and setting _PAGE_NX_BIT as well seems to be the way to
> go indeed.

Well... If you don't set the real permissions bits, how is hardware
going to know that you don't want this page to be executed, and raise a
nested page fault in the first place?

~Andrew

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