hugetlb: fix memory leak associated with vma_lock structure

The hugetlb vma_lock structure hangs off the vm_private_data pointer of
sharable hugetlb vmas.  The structure is vma specific and can not be
shared between vmas.  At fork and various other times, vmas are duplicated
via vm_area_dup().  When this happens, the pointer in the newly created
vma must be cleared and the structure reallocated.  Two hugetlb specific
routines deal with this hugetlb_dup_vma_private and hugetlb_vm_op_open. 
Both routines are called for newly created vmas.  hugetlb_dup_vma_private
would always clear the pointer and hugetlb_vm_op_open would allocate the
new vms_lock structure.  This did not work in the case of this calling
sequence pointed out in [1].

  move_vma
    copy_vma
      new_vma = vm_area_dup(vma);
      new_vma->vm_ops->open(new_vma); --> new_vma has its own vma lock.
    is_vm_hugetlb_page(vma)
      clear_vma_resv_huge_pages
        hugetlb_dup_vma_private --> vma->vm_private_data is set to NULL

When clearing hugetlb_dup_vma_private we actually leak the associated
vma_lock structure.

The vma_lock structure contains a pointer to the associated vma.  This
information can be used in hugetlb_dup_vma_private and hugetlb_vm_op_open
to ensure we only clear the vm_private_data of newly created (copied)
vmas.  In such cases, the vma->vma_lock->vma field will not point to the
vma.

Update hugetlb_dup_vma_private and hugetlb_vm_op_open to not clear
vm_private_data if vma->vma_lock->vma == vma.  Also, log a warning if
hugetlb_vm_op_open ever encounters the case where vma_lock has already
been correctly allocated for the vma.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 131a79b ("hugetlb: fix vma lock handling during split vma and range unmapping")
Signed-off-by: Mike Kravetz <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: James Houghton <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Prakash Sangappa <[email protected]>
Cc: Sven Schnelle <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: mjkravetz

mm/hugetlb.c

@@ -1014,15 +1014,23 @@ void hugetlb_dup_vma_private(struct vm_area_struct *vma)
 	VM_BUG_ON_VMA(!is_vm_hugetlb_page(vma), vma);
 	/*
 	 * Clear vm_private_data
+	 * - For shared mappings this is a per-vma semaphore that may be
+	 *   allocated in a subsequent call to hugetlb_vm_op_open.
+	 *   Before clearing, make sure pointer is not associated with vma
+	 *   as this will leak the structure.  This is the case when called
+	 *   via clear_vma_resv_huge_pages() and hugetlb_vm_op_open has already
+	 *   been called to allocate a new structure.
 	 * - For MAP_PRIVATE mappings, this is the reserve map which does
 	 *   not apply to children.  Faults generated by the children are
 	 *   not guaranteed to succeed, even if read-only.
-	 * - For shared mappings this is a per-vma semaphore that may be
-	 *   allocated in a subsequent call to hugetlb_vm_op_open.
 	 */
-	vma->vm_private_data = (void *)0;
-	if (!(vma->vm_flags & VM_MAYSHARE))
-		return;
+	if (vma->vm_flags & VM_MAYSHARE) {
+		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
+
+		if (vma_lock && vma_lock->vma != vma)
+			vma->vm_private_data = NULL;
+	} else
+		vma->vm_private_data = NULL;
 }
 
 /*
@@ -4601,6 +4609,7 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
 	struct resv_map *resv = vma_resv_map(vma);
 
 	/*
+	 * HPAGE_RESV_OWNER indicates a private mapping.
 	 * This new VMA should share its siblings reservation map if present.
 	 * The VMA will only ever have a valid reservation map pointer where
 	 * it is being copied for another still existing VMA.  As that VMA
@@ -4615,11 +4624,21 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
 
 	/*
 	 * vma_lock structure for sharable mappings is vma specific.
-	 * Clear old pointer (if copied via vm_area_dup) and create new.
+	 * Clear old pointer (if copied via vm_area_dup) and allocate
+	 * new structure.  Before clearing, make sure vma_lock is not
+	 * for this vma.
 	 */
 	if (vma->vm_flags & VM_MAYSHARE) {
-		vma->vm_private_data = NULL;
-		hugetlb_vma_lock_alloc(vma);
+		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
+
+		if (vma_lock) {
+			if (vma_lock->vma != vma) {
+				vma->vm_private_data = NULL;
+				hugetlb_vma_lock_alloc(vma);
+			} else
+				pr_warn("HugeTLB: vma_lock already exists in %s.\n", __func__);
+		} else
+			hugetlb_vma_lock_alloc(vma);
 	}
 }