[PATCH v2] xarray: clear the new head's parent before freeing the old head

[Nickolai Zeldovich <nickolai@csail.mit.edu>]

xas_shrink() passes the old head to xa_node_free(), which queues it with
call_rcu(), before it clears the new head's parent pointer.  A lockless
walker that loads the new head after the grace period has started can
still climb from it into the old head through xa_parent(), as
xas_find(), xas_find_marked(), xas_next() and xas_prev() do when they
step past the end of a node, and read the old head after it is freed.

Clear the parent pointer before freeing the old head, as
radix_tree_shrink() does.

With a delay injected after xa_node_free() in xas_shrink(), KASAN
reports a slab-use-after-free in the walker; with this patch it does
not.

Fixes: 58d6ea3085f2 ("xarray: Add XArray unconditional store operations")
---

Notes:
    This patch was prepared with the help of an AI tool.
    
    Changes in v2:
    - Shorter commit message (Matthew Wilcox).  No code change.

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

diff --git a/lib/xarray.c b/lib/xarray.c
index bfe7bef80f34e..ae9d7fc68861c 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -471,11 +471,12 @@ static void xas_shrink(struct xa_state *xas)
 		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;
 	}
 }
 

base-commit: 62c457528529cdcfa32bc52b331ceca10f3937b6
-- 
2.53.0