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

Paolo Bonzini
2014-08-19T09:09:58Z

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.

> * 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).

> + /* 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.

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

Paolo