Re: [PATCH] VT-x: simplify/clarify vmx_load_pdptrs()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.06.2020 12:11, Roger Pau Monné wrote:
> On Thu, Jun 18, 2020 at 08:38:27AM +0200, Jan Beulich wrote:
>> * Guests outside of long mode can't have PCID enabled. Drop the
>>   respective check to make more obvious that there's no security issue
>>   (from potentially accessing past the mapped page's boundary).
>>
>> * Only the low 32 bits of CR3 are relevant outside of long mode, even
>>   if they remained unchanged after leaving that mode.
>>
>> * Drop the unnecessary and badly typed local variable p.
>>
>> * Don't open-code hvm_long_mode_active() (and extend this to the related
>>   nested VT-x code).
>>
>> * Constify guest_pdptes to clarify that we're only reading from the
>>   page.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>> @@ -1312,17 +1312,16 @@ static void vmx_set_interrupt_shadow(str
>>  
>>  static void vmx_load_pdptrs(struct vcpu *v)
>>  {
>> -    unsigned long cr3 = v->arch.hvm.guest_cr[3];
>> -    uint64_t *guest_pdptes;
>> +    uint32_t cr3 = v->arch.hvm.guest_cr[3];
>> +    const uint64_t *guest_pdptes;
>>      struct page_info *page;
>>      p2m_type_t p2mt;
>> -    char *p;
>>  
>>      /* EPT needs to load PDPTRS into VMCS for PAE. */
>> -    if ( !hvm_pae_enabled(v) || (v->arch.hvm.guest_efer & EFER_LMA) )
>> +    if ( !hvm_pae_enabled(v) || hvm_long_mode_active(v) )
>>          return;
>>  
>> -    if ( (cr3 & 0x1fUL) && !hvm_pcid_enabled(v) )
>> +    if ( cr3 & 0x1f )
>>          goto crash;
> 
> Intel SDM says bits 0 to 4 are ignored, not reserved, so I'm not sure
> we need to crash the guest here. I have no reference how real hardware
> behaves here, so maybe crashing is the right thing to do.

No, I was mislead by the wording of the description in the old
patch I've referenced. IOW ...

>> @@ -1332,14 +1331,12 @@ static void vmx_load_pdptrs(struct vcpu
>>           * queue, but this is the wrong place. We're holding at least
>>           * the paging lock */
>>          gdprintk(XENLOG_ERR,
>> -                 "Bad cr3 on load pdptrs gfn %lx type %d\n",
>> +                 "Bad cr3 on load pdptrs gfn %"PRIx32" type %d\n",
>>                   cr3 >> PAGE_SHIFT, (int) p2mt);
>>          goto crash;
>>      }
>>  
>> -    p = __map_domain_page(page);
>> -
>> -    guest_pdptes = (uint64_t *)(p + (cr3 & ~PAGE_MASK));
>> +    guest_pdptes = __map_domain_page(page) + (cr3 & ~PAGE_MASK);
> 
> Instead I think this could be:
> 
> guest_pdptes = __map_domain_page(page) + (cr3 & ~(PAGE_MASK | 0x1f));

... I agree and will change to this; I'll assume your R-b stands
with the change made (and the description adjusted accordingly).

Jan