KVM: MMU: Fix host memory corruption on i386 with >= 4GB ram

PAGE_MASK is an unsigned long, so using it to mask physical addresses on
i386 (which are 64-bit wide) leads to truncation.  This can result in
page->private of unrelated memory pages being modified, with disasterous
results.

Fix by not using PAGE_MASK for physical addresses; instead calculate
the correct value directly from PAGE_SIZE.  Also fix a similar BUG_ON().
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/mmu.c

@@ -131,7 +131,7 @@ static int dbg = 1;
 	(((address) >> PT32_LEVEL_SHIFT(level)) & ((1 << PT32_LEVEL_BITS) - 1))
 
 
-#define PT64_BASE_ADDR_MASK (((1ULL << 52) - 1) & PAGE_MASK)
+#define PT64_BASE_ADDR_MASK (((1ULL << 52) - 1) & ~(u64)(PAGE_SIZE-1))
 #define PT64_DIR_BASE_ADDR_MASK \
 	(PT64_BASE_ADDR_MASK & ~((1ULL << (PAGE_SHIFT + PT64_LEVEL_BITS)) - 1))
 
@@ -406,8 +406,8 @@ static void rmap_write_protect(struct kvm_vcpu *vcpu, u64 gfn)
 			spte = desc->shadow_ptes[0];
 		}
 		BUG_ON(!spte);
-		BUG_ON((*spte & PT64_BASE_ADDR_MASK) !=
-		       page_to_pfn(page) << PAGE_SHIFT);
+		BUG_ON((*spte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT
+		       != page_to_pfn(page));
 		BUG_ON(!(*spte & PT_PRESENT_MASK));
 		BUG_ON(!(*spte & PT_WRITABLE_MASK));
 		rmap_printk("rmap_write_protect: spte %p %llx\n", spte, *spte);