mm/hmm: hmm_range_fault() NULL pointer bug

Although hmm_range_fault() calls find_vma() to make sure that a vma exists
before calling walk_page_range(), hmm_vma_walk_hole() can still be called
with walk->vma == NULL if the start and end address are not contained
within the vma range.

 hmm_range_fault() /* calls find_vma() but no range check */
  walk_page_range() /* calls find_vma(), sets walk->vma = NULL */
   __walk_page_range()
    walk_pgd_range()
     walk_p4d_range()
      walk_pud_range()
       hmm_vma_walk_hole()
        hmm_vma_walk_hole_()
         hmm_vma_do_fault()
          handle_mm_fault(vma=0)

Link: https://lore.kernel.org/r/20190823221753.2514-2-rcampbell@nvidia.comSigned-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/hmm.c

@@ -229,6 +229,9 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
 	struct vm_area_struct *vma = walk->vma;
 	vm_fault_t ret;
 
+	if (!vma)
+		goto err;
+
 	if (hmm_vma_walk->flags & HMM_FAULT_ALLOW_RETRY)
 		flags |= FAULT_FLAG_ALLOW_RETRY;
 	if (write_fault)
@@ -239,12 +242,14 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
 		/* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */
 		return -EAGAIN;
 	}
-	if (ret & VM_FAULT_ERROR) {
-		*pfn = range->values[HMM_PFN_ERROR];
-		return -EFAULT;
-	}
+	if (ret & VM_FAULT_ERROR)
+		goto err;
 
 	return -EBUSY;
+
+err:
+	*pfn = range->values[HMM_PFN_ERROR];
+	return -EFAULT;
 }
 
 static int hmm_pfns_bad(unsigned long addr,