From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from outgoing2021.csail.mit.edu (outgoing2021.csail.mit.edu [128.30.2.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B70F630D404; Mon, 12 Oct 2026 20:25:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=128.30.2.78 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791836754; cv=none; b=OV9MK8wDb9oQSTzwgwxN1ozCAGDH7Ib2+TkNXP5Wv2Dfu63/vBp/tX3D0thv+R6B27/Gtpr6lz8XlGB3cafNY4I3Yy810ybsBaQ0bCuoVmpuj/CUoKlQPAAJAD286M9lvTsgVUTFs7iRF1Mgbkb3h5cEMWTvgKTa0GZf6iwEqqE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791836754; c=relaxed/simple; bh=K/Zl5Qj1myG82TOYaKXai3M6PLPZKJnBje1SWWfefUY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=ho5Zi/kXWal+V7UkmUpFC2jlXDC71CDs3losyntiAk/8UdErFOOej5HyEbPj3GTAg5NWbRyP97UrE0+0nSb47NkFr/LrxzEmAKfiaOUZ2HL0uvWY2rS8K7NkFr3iTj/ASlg2WV5n7K+zoyHx1G/gGn1ryXD0hDN9dPn3CK0lCmM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=csail.mit.edu; spf=pass smtp.mailfrom=csail.mit.edu; arc=none smtp.client-ip=128.30.2.78 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=csail.mit.edu Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=csail.mit.edu Received: from escalante.csail.mit.edu ([128.52.128.198]) by outgoing2021.csail.mit.edu with esmtp (Exim 4.95) (envelope-from ) id 1xGMaj-00Fxjz-RA; Mon, 12 Oct 2026 16:25:49 -0400 From: Nickolai Zeldovich To: Matthew Wilcox Cc: Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Nickolai Zeldovich Subject: [PATCH v1] xarray: clear the new head's parent before freeing the old head Date: Mon, 12 Oct 2026 16:25:49 -0400 Message-ID: <20261012202549.772887-1-nickolai@csail.mit.edu> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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() 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 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. Fixes: 58d6ea3085f2 ("xarray: Add XArray unconditional store operations") Cc: stable@vger.kernel.org Assisted-by: LLM Signed-off-by: Nickolai Zeldovich --- Notes: Reproduction details: the ordering check wraps each xa_node_free(node) call in a copy of lib/xarray.c with a function that walks the tree from xa_head and reports the node if it is still reachable or still the ->parent of a reachable node. The harness race test links a copy of lib/xarray.c with a hook right after the xa_node_free() in xas_shrink() that sleeps 1 ms in the writer; 32 threads, liburcu-registered readers. Without that sleep, this patch alone still showed a use-after-free in 3 of 20 runs; those come from the xas_delete_node() bug (the race test also erases nodes), and with both patches 20 of 20 runs were clean. With only the xas_delete_node() fix, the sleep still gives 20 of 20 failures. The kernel demonstration is a test build with mdelay(100) after xa_node_free() in xas_shrink() for one test array, a writer kthread on CPU 0 doing xa_store()/xa_erase() and a reader kthread on CPU 1 doing xas_load()/xas_next() under rcu_read_lock(). The window has not been hit on an unmodified kernel. I can send any of these if useful. lib/xarray.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/lib/xarray.c b/lib/xarray.c index bfe7bef80f34..ae9d7fc68861 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: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907 -- 2.53.0