vfio/type1: Use follow_pte()

follow_pfn() doesn't make sure that we're using the correct page
protections, get the pte with follow_pte() so that we can test
protections and get the pfn from the pte.

Fixes: 5cbf3264 ("vfio/type1: Fix VA->PA translation for PFNMAP VMAs in vaddr_get_pfn()")
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_type1.c

@@ -24,6 +24,7 @@
 #include <linux/compat.h>
 #include <linux/device.h>
 #include <linux/fs.h>
+#include <linux/highmem.h>
 #include <linux/iommu.h>
 #include <linux/module.h>
 #include <linux/mm.h>
@@ -462,9 +463,11 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
 			    unsigned long vaddr, unsigned long *pfn,
 			    bool write_fault)
 {
+	pte_t *ptep;
+	spinlock_t *ptl;
 	int ret;
 
-	ret = follow_pfn(vma, vaddr, pfn);
+	ret = follow_pte(vma->vm_mm, vaddr, &ptep, &ptl);
 	if (ret) {
 		bool unlocked = false;
 
@@ -478,9 +481,17 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
 		if (ret)
 			return ret;
 
-		ret = follow_pfn(vma, vaddr, pfn);
+		ret = follow_pte(vma->vm_mm, vaddr, &ptep, &ptl);
+		if (ret)
+			return ret;
 	}
 
+	if (write_fault && !pte_write(*ptep))
+		ret = -EFAULT;
+	else
+		*pfn = pte_pfn(*ptep);
+
+	pte_unmap_unlock(ptep, ptl);
 	return ret;
 }