Re: [PATCH v2 1/3] KVM: vmx: fix ept reserved bits for 1-GByte page

Wanpeng Li
2014-08-19T11:15:14Z

On Tue, Aug 19, 2014 at 11:09:49AM +0200, Paolo Bonzini wrote:
>Il 19/08/2014 11:04, Wanpeng Li ha scritto:
>> EPT misconfig handler in kvm will check which reason lead to EPT
>> misconfiguration after vmexit. One of the reasons is that an EPT
>> paging-structure entry is configured with settings reserved for
>> future functionality. However, the handler can't identify if
>> paging-structure entry of reserved bits for 1-GByte page are
>> configured, since PDPTE which point to 1-GByte page will reserve
>> bits 29:12 instead of bits 7:3 which are reserved for PDPTE that
>> references an EPT Page Directory. This patch fix it by reserve
>> bits 29:12 for 1-GByte page.
>>
>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>> ---
>> v1 -> v2:
>> * same "if" statement cover both 2MB and 1GB pages
>> * return 0xf8 for level == 4
>
>I think you dropped this check by mistake.

Indeed. I will do it in next version.

>
>> * get the level by checking the return value of ept_rsvd_mask
>>
>> arch/x86/kvm/vmx.c | 19 +++++++------------
>> 1 file changed, 7 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index cad37d5..2763f37 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -5521,17 +5521,12 @@ static u64 ept_rsvd_mask(u64 spte, int level)
>> for (i = 51; i > boot_cpu_data.x86_phys_bits; i--)
>> mask |= (1ULL << i);
>>
>> - if (level > 2)
>> - /* bits 7:3 reserved */
>> - mask |= 0xf8;
>> - else if (level == 2) {
>> - if (spte & (1ULL << 7))
>> - /* 2MB ref, bits 20:12 reserved */
>> - mask |= 0x1ff000;
>> - else
>> - /* bits 6:3 reserved */
>> - mask |= 0x78;
>> - }
>> + if (spte & (1ULL << 7))
>
>You need to go this way if level == 1 too. Otherwise, you would report
>bits 6:3 reserved if the hypervisor is using the ignored bit 7 (Table
>28-6, Format of an EPT Page-Table Entry).
>

Agreed. What still need to do here is to update the comments in order to
include level == 1, right?

>> + /* 1GB/2MB page, bits 29:12 or 20:12 reserved respectively */
>> + mask |= (PAGE_SIZE << ((level - 1) * 9)) - PAGE_SIZE;
>> + else
>> + /* bits 6:3 reserved */
>> + mask |= 0x78;
>>
>> return mask;
>> }
>> @@ -5561,7 +5556,7 @@ static void ept_misconfig_inspect_spte(struct kvm_vcpu *vcpu, u64 spte,
>> WARN_ON(1);
>> }
>>
>> - if (level == 1 || (level == 2 && (spte & (1ULL << 7)))) {
>> + if (level == 1 || (rsvd_bits & 0x38)) {
>
>- rsvd_bits will always be zero here. You need to check the return
>value of ept_rsvd_mask(). Let's call it rsvd_mask in the rest of this
>email.
>
>- the test is inverted, you need to check that bits 5:3 are _not_
>reserved, hence (rsvd_mask & 0x38) == 0.
>
>- once you do this, the test also covers level 1.

Agreed.

>
>I suggest that you write a testcase for kvm-unit-tests.
>

Ok.

Regards,
Wanpeng Li

>Paolo