Page fault in page walking

[cyruscyliu]

In vmcs_linear2phy()

bool vmcs_linear2phy(bx_address laddr, bx_phy_address *phy)
{
      // ...
      for (; level >= 0; --level) {
        Bit64u pte;
        pt_address += ((laddr >> (9 + 9*level)) & 0xff8);
        offset_mask >>= 9;
        if (vmcs_translate_guest_physical_ept(pt_address, &pt_address, NULL))
            goto page_fault;
        BX_MEM(0)->readPhysicalPage(BX_CPU(id), pt_address, 8, &pte);    // <<<<<<<<<<<<<<<<<<<<<<<<<<
        if(!(pte & 1))
          goto page_fault;
        if (pte & BX_PAGING_PHY_ADDRESS_RESERVED_BITS)
          goto page_fault;
        pt_address = bx_phy_address(pte & BX_CONST64(0x000ffffffffff000));
        if (level == BX_LEVEL_PTE) break;
        if (pte & 0x80) {
          // large page
          pt_address &= BX_CONST64(0x000fffffffffe000);
          if (pt_address & offset_mask)
            goto page_fault;
          if (BX_CPU(id)->is_cpu_extension_supported(BX_ISA_1G_PAGES) && level == BX_LEVEL_PDPTE) break;
          if (level == BX_LEVEL_PDE) break;
          goto page_fault;
        }
      }

// <<<<<<<<<<<<<<<<<<<<<<<<<<
pte will be 0, which implies a page fault

But when pte is zero, this shouldn't crash HyperPill. It should go to page_fault and exit.

I found it was due to the bad compiler. Clang will remove the checks below with O2 and O3

        if(!(pte & 1))
          goto page_fault;
        if (pte & BX_PAGING_PHY_ADDRESS_RESERVED_BITS)
          goto page_fault;

[cyruscyliu]

Compiling HyperPill with O0 and O1 won't remove these checks.

[cyruscyliu]

a workaround to keep the checks

diff --git a/ept.cc b/ept.cc
index 056db22..2d9b223 100644
--- a/ept.cc
+++ b/ept.cc
@@ -128,6 +128,7 @@ int vmcs_translate_guest_physical_ept(bx_phy_address guest_paddr, bx_phy_address
 }
 
 // Translate GVA -> GPA -> HPA
+__attribute__((optnone))
 bool vmcs_linear2phy(bx_address laddr, bx_phy_address *phy)
 {
   bx_phy_address paddress;

[cyruscyliu]

I tested clang-11.1.0-6, 12.0.1-19ubuntu3, 13.0.1-2ubuntu2.2, and 14.0.0-1ubuntu1.1, they all keep the checks.
But, clang 15.0.7 removes the checks.