From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 66F123D9DC0; Tue, 1 Sep 2026 03:08:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788232103; cv=none; b=E+sybSdxyyxa2GVXcRNdg6yDYINdWz35+163Ug9ZggRovQysOzfhehC8zVH2XitdMxBzuD3YH0Gi42kPakjVXkv6QoLzneNQodBWC5sBeIqaAA97w3qyYghJh9VbsnvNeriWNckeG0B2bCjnZjGtrgRKNRBwKZbVpXDAOLd8NtQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788232103; c=relaxed/simple; bh=wTAj7j+kfNNe9TFWu03zyquPjDfJPBnNKzuGVDeLdvs=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=nKt++G1OMs5mnsHs9AGuYBn+fO3Y9FLaa5ZaZAMRDS5LXMqUTb6MLRuqfyH97tByDyHNlDNLnmkFAb3DttjPlONRH1kzEYVOISrov0VfHwV2tcoTqxxl5uBwvdBg7cMI9A7rKx8ghOxuYWvNQ+/dyVpNSEMFYxY32l0Bcbqolus= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jadh3hiV; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jadh3hiV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 88C191F000E9; Tue, 1 Sep 2026 03:08:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788232102; bh=mHanEbP43xNR3nBIqo5iaqZoua+wJSDpVuuuXC39M64=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=jadh3hiVAG6cNlr3l1JQOYzB13QBnzc0O7TKsT0mFRXthaAX7WKv6k3nAaQL8jVhU zyJNbkSYJGusEkiDw+OUNTlvcBYfZL+MCUPAvrBjCwYfL+scy28wGqcBDF/FOvHais tBz/9WcTxj9LbgJP2yPvAUCKOusw8D27Ba+KgSJlY58Uvq2n2xdmWi76y0pllGTLLu xnFjj+lVTy0aKq0TUlvBZ8xjoUNxcaOnDsJKQPnWcyJGFKslr9YUNMHwC2tXHwT14s QpWLOOLEXEZRLGNhaRXj2BVkWHHxuAKcDYIN5sCBCcWTqPF5HEkr/is/8mrqu50Uql RxVtGRlBti4ag== Date: Mon, 31 Aug 2026 20:08:20 -0700 From: Jakub Kicinski To: Jamal Hadi Salim Cc: netdev@vger.kernel.org, Jiri Pirko , "David S. Miller" , Eric Dumazet , Paolo Abeni , Simon Horman , Willem de Bruijn , Jason Wang , Andrew Lunn , stable@vger.kernel.org, vega@nebusec.ai, Victor Nogueira Subject: Re: [PATCH net 1/2] net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations Message-ID: <20260831200820.523ac597@kernel.org> In-Reply-To: <20260828121902.66837-1-jhs@mojatatu.com> References: <20260828121902.66837-1-jhs@mojatatu.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 28 Aug 2026 08:19:01 -0400 Jamal Hadi Salim wrote: > Several subsystems allocate ring buffers sized by dev->tx_queue_len > with no upper bound. An unprivileged user (via unshare -Urn) can set a > huge tx_queue_len and create many devices/queues to exhaust global > memory, causing a system-wide OOM: > > - pfifo_fast: pfifo_fast_init() and pfifo_fast_change_tx_queue_len() > allocate 3 skb_array rings of tx_queue_len entries each. > - tun: tun_queue_resize() and the queue-attach path resize ptr_rings > to tx_queue_len on the NETDEV_CHANGE_TX_QUEUE_LEN notifier. > - tap (macvtap/ipvtap): tap_queue_resize() and tap_init() resize/init > ptr_rings to tx_queue_len on the same notifier. > > netif_change_tx_queue_len() is the single entry point for IFLA_TXQLEN, > sysfs, and the SIOCSIFTXQLEN ioctl. Cap new_len at S16_MAX (32767) > there so the oversized value is rejected at set time. This will take > effect whether the device is up or down before dev->tx_queue_len is > written or any notifier fires or any ring is allocated. So a good > choke spot. > > S16_MAX is the virtio virtqueue size limit: the virtio specification > stores the queue size as a u16 with a maximum of 32768, so 32767 is > the largest tx_queue_len any in-tree driver can meaningfully use. > Values above that only serve to inflate ring allocations. > > Conditions to recreate the bug: > - CONFIG_NET_SCHED=y, CONFIG_VETH=y, CONFIG_USER_NS=y, CONFIG_NET_NS=y. > - Unprivileged user in a fresh user+net namespace (unshare -Urn). > - pfifo_fast: create veth pairs, set tx_queue_len to 500000, attach > mq+pfifo_fast. ~28 iterations OOMs a 2GB guest. > - tun: create 50 tun devices with IFF_MULTI_QUEUE, set tx_queue_len to > 500000, open 8 queues each. ~1.6GB of ptr_ring allocations OOMs a > 512MB guest. > - tap: same as tun with IFF_TAP. ~960MB OOMs a 512MB guest. > - On the fixed kernel the oversized tx_queue_len is rejected with > -ERANGE at set time. > > Fixes: 6a643ddb5624 ("net: introduce helper dev_change_tx_queue_len()") > Reported-by: vega@nebusec.ai > Signed-off-by: Jamal Hadi Salim > --- > net/core/dev.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/net/core/dev.c b/net/core/dev.c > index 38336858c168..1d3fc0a268a5 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -9982,7 +9982,7 @@ int netif_change_tx_queue_len(struct net_device *dev, unsigned long new_len) > unsigned int orig_len = dev->tx_queue_len; > int res; > > - if (new_len != (unsigned int)new_len) > + if (new_len > S16_MAX) > return -ERANGE; > > if (new_len != orig_len) { clashiko points out that we need to also cover the newlink path: net/core/rtnetlink.c:rtnl_create_link() { ... if (tb[IFLA_TXQLEN]) dev->tx_queue_len = nla_get_u32(tb[IFLA_TXQLEN]); ...