From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 69E06C432C3 for ; Mon, 18 Nov 2019 20:49:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3E289222A5 for ; Mon, 18 Nov 2019 20:49:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=hartkopp.net header.i=@hartkopp.net header.b="r8lPzX2f" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726909AbfKRUtj (ORCPT ); Mon, 18 Nov 2019 15:49:39 -0500 Received: from mo4-p01-ob.smtp.rzone.de ([85.215.255.53]:15666 "EHLO mo4-p01-ob.smtp.rzone.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726536AbfKRUtj (ORCPT ); Mon, 18 Nov 2019 15:49:39 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; t=1574110177; s=strato-dkim-0002; d=hartkopp.net; h=In-Reply-To:Date:Message-ID:From:References:To:Subject: X-RZG-CLASS-ID:X-RZG-AUTH:From:Subject:Sender; bh=r/rBuQ55oLDSkHY0eP214V9Mlwgvx9I0t5GHGWN+1pY=; b=r8lPzX2f0wprGR/JauHOU/N3GHwjFlgHzbioKwSVcmz5SS6ZQc03l/LnYAQqPlUUT4 u3PTRtqGA6w7hT2iI24RO1KZNR7jeCneojGjKHcFZtzi/rVXiilhutzrOX3Sbe/T9MQ6 mUdmyPGEKQMqLm9kPXb7OsM4HatRTlI/PUkE2958MQAK6zCukuOx9rRDWJMhmiou0OsX 0scd5BhLG1hjV5j+BBDOMeBy0ZtGHNtJMCCe0ca39zyvAOC8jbm3lCZ59mDXiUgQ99Yg Rg3MDdOQSfSaf0to7UxYrrDlXgps1PEzmJO+68y/Jzqc2Opcs3AUr9UTZkSNBvBtwvfT lcAA== X-RZG-AUTH: ":P2MHfkW8eP4Mre39l357AZT/I7AY/7nT2yrDxb8mjG14FZxedJy6qgO1o3PMaViOoLMJV8h+lCA=" X-RZG-CLASS-ID: mo00 Received: from [192.168.1.177] by smtp.strato.de (RZmta 44.29.0 DYNA|AUTH) with ESMTPSA id C03a03vAIKnV19G (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (curve secp521r1 with 521 ECDH bits, eq. 15360 bits RSA)) (Client did not present a certificate); Mon, 18 Nov 2019 21:49:31 +0100 (CET) Subject: Re: KMSAN: uninit-value in can_receive To: Marc Kleine-Budde , syzbot , davem@davemloft.net, glider@google.com, linux-can@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com References: <0000000000005c08d10597a3a05d@google.com> From: Oliver Hartkopp Message-ID: <7934bc2b-597f-0bb3-be2d-32f3b07b4de9@hartkopp.net> Date: Mon, 18 Nov 2019 21:49:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 18/11/2019 21.29, Marc Kleine-Budde wrote: > On 11/18/19 9:25 PM, Oliver Hartkopp wrote: >>> IMPORTANT: if you fix the bug, please add the following tag to the commit: >>> Reported-by: syzbot+b02ff0707a97e4e79ebb@syzkaller.appspotmail.com >>> >>> ===================================================== >>> BUG: KMSAN: uninit-value in can_receive+0x23c/0x5e0 net/can/af_can.c:649 >>> CPU: 1 PID: 3490 Comm: syz-executor.2 Not tainted 5.4.0-rc5+ #0 >> >> In line 649 of 5.4.0-rc5+ we can find a while() statement: >> >> while (!(can_skb_prv(skb)->skbcnt)) >> can_skb_prv(skb)->skbcnt = atomic_inc_return(&skbcounter); >> >> In linux/include/linux/can/skb.h we see: >> >> static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb) >> { >> return (struct can_skb_priv *)(skb->head); >> } >> >> IMO accessing can_skb_prv(skb)->skbcnt at this point is a valid >> operation which has no uninitialized value. >> >> Can this probably be a false positive of KMSAN? > > The packet is injected via the packet socket into the kernel. Where does > skb->head point to in this case? When the skb is a proper > kernel-generated skb containing a CAN-2.0 or CAN-FD frame skb->head is > maybe properly initialized? The packet is either received via vcan or vxcan which checks via can_dropped_invalid_skb() if we have a valid ETH_P_CAN type skb. We additionally might think about introducing a check whether we have a can_skb_reserve() created skbuff. But even if someone forged a skbuff without this reserved space the access to can_skb_prv(skb)->skbcnt would point into some CAN frame content - which is still no access to uninitialized content, right? Regards, Oliver