mm/mempolicy: fix mbind_range() arguments to vma_merge()

Fuzzing produced an invalid argument to vma_merge() which was caught by
the newly added verification of the number of VMAs being removed on
process exit.  Analyzing the failure eventually resulted in finding an
issue with the search of a VMA that started at address 0, which caused an
underflow and thus the loss of many VMAs being tracked in the tree.  Fix
the underflow by changing the search of the maple tree to use the start
address directly.

Link: https://lkml.kernel.org/r/20221015021135.2816178-1-Liam.Howlett@oracle.com
Fixes: 66850be5 ("mm/mempolicy: use vma iterator & maple state instead of vma linked list")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
  Link: https://lore.kernel.org/r/202210052318.5ad10912-oliver.sang@intel.com
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mempolicy.c

@@ -787,17 +787,22 @@ static int vma_replace_policy(struct vm_area_struct *vma,
 static int mbind_range(struct mm_struct *mm, unsigned long start,
 		       unsigned long end, struct mempolicy *new_pol)
 {
-	MA_STATE(mas, &mm->mm_mt, start - 1, start - 1);
+	MA_STATE(mas, &mm->mm_mt, start, start);
 	struct vm_area_struct *prev;
 	struct vm_area_struct *vma;
 	int err = 0;
 	pgoff_t pgoff;
 
-	prev = mas_find_rev(&mas, 0);
-	if (prev && (start < prev->vm_end))
-		vma = prev;
-	else
-		vma = mas_next(&mas, end - 1);
+	prev = mas_prev(&mas, 0);
+	if (unlikely(!prev))
+		mas_set(&mas, start);
+
+	vma = mas_find(&mas, end - 1);
+	if (WARN_ON(!vma))
+		return 0;
+
+	if (start > vma->vm_start)
+		prev = vma;
 
 	for (; vma; vma = mas_next(&mas, end - 1)) {
 		unsigned long vmstart = max(start, vma->vm_start);