[PATCH 3/5] mmu: don't set the present bit unconditionally

Bandan Das
2016-06-28T04:32:38Z

To support execute only mappings on behalf of L1
hypervisors, we teach set_spte() to honor L1's valid XWR
bits. This is only if host supports EPT execute only. Reuse
ACC_USER_MASK to signify if the L1 hypervisor has the R bit
set

Signed-off-by: Bandan Das <bsd@redhat.com>
---
arch/x86/kvm/mmu.c | 9 +++++++--
arch/x86/kvm/paging_tmpl.h | 2 +-
arch/x86/kvm/vmx.c | 2 +-
3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 875d4f7..ee2fb16 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2516,13 +2516,17 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
gfn_t gfn, kvm_pfn_t pfn, bool speculative,
bool can_unsync, bool host_writable)
{
- u64 spte;
+ u64 spte = 0;
int ret = 0;
+ struct kvm_mmu *context = &vcpu->arch.mmu;
+ bool execonly = !(context->guest_rsvd_check.bad_mt_xwr &
+ (1ull << VMX_EPT_EXECUTABLE_MASK));

if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
return 0;

- spte = PT_PRESENT_MASK;
+ if (!execonly)
+ spte |= PT_PRESENT_MASK;
if (!speculative)
spte |= shadow_accessed_mask;

@@ -2531,6 +2535,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
else
spte |= shadow_nx_mask;

+ /* In the EPT case, shadow_user_mask is PT_PRESENT_MASK */
if (pte_access & ACC_USER_MASK)
spte |= shadow_user_mask;

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index bc019f7..896118e 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -187,7 +187,7 @@ static inline unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, u64 gpte)
#if PTTYPE == PTTYPE_EPT
access = ((gpte & VMX_EPT_WRITABLE_MASK) ? ACC_WRITE_MASK : 0) |
((gpte & VMX_EPT_EXECUTABLE_MASK) ? ACC_EXEC_MASK : 0) |
- ACC_USER_MASK;
+ ((gpte & VMX_EPT_READABLE_MASK) ? ACC_USER_MASK : 0);
#else
BUILD_BUG_ON(ACC_EXEC_MASK != PT_PRESENT_MASK);
BUILD_BUG_ON(ACC_EXEC_MASK != 1);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 003618e..417debc 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6366,7 +6366,7 @@ static __init int hardware_setup(void)
vmx_disable_intercept_msr_write_x2apic(0x83f);

if (enable_ept) {
- kvm_mmu_set_mask_ptes(0ull,
+ kvm_mmu_set_mask_ptes(PT_PRESENT_MASK,
(enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
(enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
0ull, VMX_EPT_EXECUTABLE_MASK);
--
2.5.5