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* + xarray-clear-the-new-heads-parent-before-freeing-the-old-head.patch added to mm-unstable branch
@ 2026-10-23 18:11 Andrew Morton
  2026-10-27 17:41 ` Matthew Wilcox
  0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2026-10-23 18:11 UTC (permalink / raw)
  To: mm-commits, willy, stable, nickolai, akpm


The patch titled
     Subject: xarray: clear the new head's parent before freeing the old head
has been added to the -mm mm-unstable branch.  Its filename is
     xarray-clear-the-new-heads-parent-before-freeing-the-old-head.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/xarray-clear-the-new-heads-parent-before-freeing-the-old-head.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nickolai Zeldovich <nickolai@csail.mit.edu>
Subject: xarray: clear the new head's parent before freeing the old head
Date: Mon, 12 Oct 2026 16:25:49 -0400

When xas_shrink() replaces the head node of an array with the node in its
slot 0, it stores the child in xa_head, passes the old head to
xa_node_free(), which queues it with call_rcu(), and only then clears the
child's ->parent pointer.  In between, a lockless walker can still reach
the old head, not from xa_head but upwards from the new head: xas_find(),
xas_find_marked(), xas_next() and xas_prev() climb through xa_parent()
when they step past the end of a node.  RCU does not protect a walker that
gets there: call_rcu() waits only for the read-side critical sections that
were running when the callback's grace period started, and that grace
period can start before the parent pointer is cleared.

  CPU 0: xa_erase(), xa_lock held   CPU 1: RCU walker, e.g. xa_find()

  xas_shrink()
    xa->xa_head = child
    xa_node_free(old head)
      call_rcu()
                    <grace period starts>
                                     passes a quiescent state
                                     rcu_read_lock()
                                     xas_find(): loads xa_head (child),
                                     steps past its last slot,
                                     child->parent is still old head
    child->parent = NULL
  xa_unlock()
  quiescent state
                    <grace period ends>
  radix_tree_node_rcu_free(old head)
                                     reads old head's slots[], shift,
                                     offset and parent

The grace period does not wait for CPU 1, which reported its quiescent
state before its critical section began; holding xa_lock delays the end of
the grace period, not its start.  The start can fall inside the window
when an interrupt arrives in it (interrupts are enabled under xa_lock(),
and RCU core processing may run on interrupt exit), when callbacks are
offloaded (rcu_nocbs=) and the rcuog kthread on another CPU handles the
callback, when many callbacks are queued, or while the writer is preempted
under PREEMPT_RT.  Interrupts, and in a guest a preempted vCPU, also
stretch the window.

After the climb, xas_find(), xas_find_marked() and xas_next() resume in
the old head at the slot after the child and read its slots[] and shift
(xas_find_marked() also its mark bitmaps); xas_prev(), for which the child
is the first slot, steps past the old head at once and reads its offset
and parent, as the others do when they reach its end.  Nothing in the walk
notices that the node is gone.  The climb takes ->parent as it is, no
lockless path looks at node->array (XA_RCU_FREE), and xas_shrink() stores
XA_RETRY_ENTRY in slot 0 of the old head only when the head shrinks to a
single entry, not to a node; the walker does not look at slot 0 anyway. 
radix_tree_node_cachep is not SLAB_TYPESAFE_BY_RCU, so the walker reads
freed memory.  Once the object is reused for a node of another array, the
walker returns that array's entries or descends and climbs into its nodes.

The order dates from the introduction of xas_shrink() in the commit below,
which reimplemented radix_tree_shrink().  radix_tree_shrink() clears the
child's parent pointer before it replaces the head and frees the old node,
and still does in lib/radix-tree.c.  The commit gives no reason for the
different order and nothing depends on it: xas_update() and xa_node_free()
only touch the old head.

Clear the child's parent pointer before freeing the old head, as
radix_tree_shrink() does.  The stores are the same, the tree and the
xa_state the caller sees are the same, and a walker that starts after the
callback's grace period has begun sees the cleared pointer, like every
store before call_rcu(), and ends its walk at the new head, which covers
every index that the old head had an entry for.

A check in the radix-tree test harness that no node passed to
xa_node_free() is still the parent of a node reachable from xa_head fails
in xas_shrink() 4731 times in the XArray tests without this patch and
never with it.  No test is added: the race is not deterministic and the
check needs a hook on xa_node_free().  A harness race test (readers
walking the whole array with the four climbing walkers while a writer
stores and erases an entry that adds three levels above the leaf) did not
hit the window in 20 runs of 200000 erases; with a 1 ms sleep injected
after xa_node_free() in xas_shrink(), every one of 20 runs reported a
heap-use-after-free in __xas_next() or __xas_prev() under
AddressSanitizer, and none did with this patch and the same sleep.  On a
kernel (x86_64 defconfig + KASAN, QEMU), with a 100 ms delay injected at
the same place, a walk that starts on another CPU 20 ms into the window
climbs to the old head in each of 20 rounds, and KASAN reports a
slab-use-after-free of the radix_tree_node freed from rcu_core() when the
walk takes its next step; with this patch, the walk ends at the new head. 
The XArray and IDA self-tests pass on x86_64 and riscv64.

Found while formally verifying xas_store() against the kernel binary.

Link: https://lore.kernel.org/20261012202549.772887-1-nickolai@csail.mit.edu
Fixes: 58d6ea3085f2 ("xarray: Add XArray unconditional store operations")
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Assisted-by: LLM
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
---

 lib/xarray.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/lib/xarray.c~xarray-clear-the-new-heads-parent-before-freeing-the-old-head
+++ a/lib/xarray.c
@@ -471,11 +471,12 @@ static void xas_shrink(struct xa_state *
 		if (!xa_is_node(entry))
 			RCU_INIT_POINTER(node->slots[0], XA_RETRY_ENTRY);
 		xas_update(xas, node);
+		if (xa_is_node(entry))
+			xa_to_node(entry)->parent = NULL;
 		xa_node_free(node);
 		if (!xa_is_node(entry))
 			break;
 		node = xa_to_node(entry);
-		node->parent = NULL;
 	}
 }
 
_

Patches currently in -mm which might be from nickolai@csail.mit.edu are

mm-mlock-use-a-long-for-the-page-count-in-mlock_fixup.patch
mm-truncate-fix-the-zeroing-end-of-pagecache_isize_extended.patch
resource-report-a-devm_request_resource-conflict-under-the-lock.patch
mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch
xarray-clear-the-new-heads-parent-before-freeing-the-old-head.patch
mm-vmalloc-read-vm_map_ram-block-regions-from-the-requested-offset.patch


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2026-10-23 18:11 + xarray-clear-the-new-heads-parent-before-freeing-the-old-head.patch added to mm-unstable branch Andrew Morton
2026-10-27 17:41 ` Matthew Wilcox
2026-10-27 18:13   ` Andrew Morton

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