From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 86C933568E7; Tue, 26 Aug 2025 14:29:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756218541; cv=none; b=MtLQT3BHbIwBsDNmSn231psUrBG470Gvlj4YUmKewgebZJdHCKWDUa+TFRg7keboDhded/IxcFNqLFnxusWmtVbWYpSJu59LbiAoZS+5tZg/tt1aprew9mY2qntTkgZYb5OlyjIbhNPE2mh5VHKWmL6vl04aKXNX58dbOfz7FmI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756218541; c=relaxed/simple; bh=vS+ROOjklo/VJNQ3+F7pGYJZ/zyFAknUF2N/gcPZZ5k=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=nZ/gcIWhhSBl7f3jymw9NTGkCu4S+gmYCxnxYfi/8xn3UbTAFUtQT5ZXnErzP75L4kfSiPWQcfdx5w6Q+cSmoFd2wWpVSAl/5EX10PrtqzBkMEZRchzjaAyolgCdk8OmCa6qGylHLZ4eDlw0NRkvh5EojIy8QkoT+l2bglOeQHk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=dpaklglL; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="dpaklglL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1E1AFC4CEF1; Tue, 26 Aug 2025 14:29:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1756218541; bh=vS+ROOjklo/VJNQ3+F7pGYJZ/zyFAknUF2N/gcPZZ5k=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=dpaklglLp82WPChR0JFUt/PysolOK0N9CIy1w1bjrrvxW0pkEYHu35diRsQ8D88Bp WSmICIBh4VRo8YExz3r+kwroGX9e0RrriRzt1S1zsvsO3e8KI6cDMj5K/p5NEpjIPv 7jeELLv6AWzsZnxUwKUcjlKV9AmfvZA3XhZf4of4= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, William Liu , Savino Dicanosa , Jakub Kicinski , Sasha Levin Subject: [PATCH 5.4 039/403] net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree Date: Tue, 26 Aug 2025 13:06:05 +0200 Message-ID: <20250826110906.898579547@linuxfoundation.org> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20250826110905.607690791@linuxfoundation.org> References: <20250826110905.607690791@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: William Liu [ Upstream commit 0e1d5d9b5c5966e2e42e298670808590db5ed628 ] htb_lookup_leaf has a BUG_ON that can trigger with the following: tc qdisc del dev lo root tc qdisc add dev lo root handle 1: htb default 1 tc class add dev lo parent 1: classid 1:1 htb rate 64bit tc qdisc add dev lo parent 1:1 handle 2: netem tc qdisc add dev lo parent 2:1 handle 3: blackhole ping -I lo -c1 -W0.001 127.0.0.1 The root cause is the following: 1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on the selected leaf qdisc 2. netem_dequeue calls enqueue on the child qdisc 3. blackhole_enqueue drops the packet and returns a value that is not just NET_XMIT_SUCCESS 4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate -> htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase 5. As this is the only class in the selected hprio rbtree, __rb_change_child in __rb_erase_augmented sets the rb_root pointer to NULL 6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL, which causes htb_dequeue_tree to call htb_lookup_leaf with the same hprio rbtree, and fail the BUG_ON The function graph for this scenario is shown here: 0) | htb_enqueue() { 0) + 13.635 us | netem_enqueue(); 0) 4.719 us | htb_activate_prios(); 0) # 2249.199 us | } 0) | htb_dequeue() { 0) 2.355 us | htb_lookup_leaf(); 0) | netem_dequeue() { 0) + 11.061 us | blackhole_enqueue(); 0) | qdisc_tree_reduce_backlog() { 0) | qdisc_lookup_rcu() { 0) 1.873 us | qdisc_match_from_root(); 0) 6.292 us | } 0) 1.894 us | htb_search(); 0) | htb_qlen_notify() { 0) 2.655 us | htb_deactivate_prios(); 0) 6.933 us | } 0) + 25.227 us | } 0) 1.983 us | blackhole_dequeue(); 0) + 86.553 us | } 0) # 2932.761 us | qdisc_warn_nonwc(); 0) | htb_lookup_leaf() { 0) | BUG_ON(); ------------------------------------------ The full original bug report can be seen here [1]. We can fix this just by returning NULL instead of the BUG_ON, as htb_dequeue_tree returns NULL when htb_lookup_leaf returns NULL. [1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/ Fixes: 512bb43eb542 ("pkt_sched: sch_htb: Optimize WARN_ONs in htb_dequeue_tree() etc.") Signed-off-by: William Liu Signed-off-by: Savino Dicanosa Link: https://patch.msgid.link/20250717022816.221364-1-will@willsroot.io Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/sched/sch_htb.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) --- a/net/sched/sch_htb.c +++ b/net/sched/sch_htb.c @@ -776,7 +776,9 @@ static struct htb_class *htb_lookup_leaf u32 *pid; } stk[TC_HTB_MAXDEPTH], *sp = stk; - BUG_ON(!hprio->row.rb_node); + if (unlikely(!hprio->row.rb_node)) + return NULL; + sp->root = hprio->row.rb_node; sp->pptr = &hprio->ptr; sp->pid = &hprio->last_ptr_id;