KVM: MMU: Fix potential race setting upper shadow ptes on nonpae hosts

The direct mapped shadow code (used for real mode and two dimensional paging)
sets upper-level ptes using direct assignment rather than calling
set_shadow_pte().  A nonpae host will split this into two writes, which opens
up a race if another vcpu accesses the same memory area.

Fix by calling set_shadow_pte() instead of assigning directly.

Noticed by Izik Eidus.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7f57da663826..b0e4ddca6c18 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1189,9 +1189,10 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
                                 return -ENOMEM;
                         }
 
-                        table[index] = __pa(new_table->spt)
-                                | PT_PRESENT_MASK | PT_WRITABLE_MASK
-                                | shadow_user_mask | shadow_x_mask;
+                        set_shadow_pte(&table[index],
+                                       __pa(new_table->spt)
+                                       | PT_PRESENT_MASK | PT_WRITABLE_MASK
+                                       | shadow_user_mask | shadow_x_mask);
                 }
                 table_addr = table[index] & PT64_BASE_ADDR_MASK;
         }