kvm, ept: remove the default write bit

When ept enabled, current code set shadow_base_present_pte including the write bit, thus all pte entries have writabe bit, and it means guest os can always write to any mapped page (even VMM maps RO pages for the guest.)

We always use get_user_pages(write=1), so this bad code does not cause any bad result currently.

But it is really bad, so fix it, and we will use RO pages future.

We will set writabe bit when it is really writable (determined by the parameters of the set_spte())

Signed-off-by Lai Jiangshan <[email protected]>

---

arch/x86/kvm/vmx.c +0 -0

index fdb18cf..c7565ea 100644

Quoted 22 lines · Lai Jiangshan

When ept enabled, current code set shadow_base_present_pte including the write bit, thus all pte entries have writabe bit, and it means guest os can always write to any mapped page (even VMM maps RO pages for the guest.)

We always use get_user_pages(write=1), so this bad code does not cause any bad result currently.

But it is really bad, so fix it, and we will use RO pages future.

We will set writabe bit when it is really writable (determined by the parameters of the set_spte())

Signed-off-by Lai Jiangshan<[email protected]>

---

arch/x86/kvm/vmx.c +0 -0

index fdb18cf..c7565ea 100644
@@ -4365,8 +4365,7 @@ static int __init vmx_init(void)
 

arch/x86/kvm/vmx.c static int __init vmx_init(void) · line 4371

Quoted 9 lines

arch/x86/kvm/vmx.c +1 -2

  	if (enable_ept) {
  		bypass_guest_pf = 0;
-		kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK |
-			VMX_EPT_WRITABLE_MASK);
+		kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK);
  		kvm_mmu_set_mask_ptes(0ull, 0ull, 0ull, 0ull,
  				VMX_EPT_EXECUTABLE_MASK);
  		kvm_enable_tdp();

You can remove the call to kvm_mmu_set_base_ptes entirely, because VMX_EPT_READABLE_MASK == PT_PRESENT_MASK.