mm: fix madvise infinine loop

madvise(MADV_REMOVE) can go into an infinite loop or cause an oops if the
call covers a region from the start of a vma, and extending past that vma.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

mm/madvise.c

@@ -155,11 +155,14 @@ static long madvise_dontneed(struct vm_area_struct * vma,
  * Other filesystems return -ENOSYS.
  */
 static long madvise_remove(struct vm_area_struct *vma,
+				struct vm_area_struct **prev,
 				unsigned long start, unsigned long end)
 {
 	struct address_space *mapping;
         loff_t offset, endoff;
 
+	*prev = vma;
+
 	if (vma->vm_flags & (VM_LOCKED|VM_NONLINEAR|VM_HUGETLB))
 		return -EINVAL;
 
@@ -199,7 +202,7 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
 		error = madvise_behavior(vma, prev, start, end, behavior);
 		break;
 	case MADV_REMOVE:
-		error = madvise_remove(vma, start, end);
+		error = madvise_remove(vma, prev, start, end);
 		break;
 
 	case MADV_WILLNEED: