From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oliver Hartkopp Subject: Re: KMSAN: uninit-value in can_receive Date: Mon, 18 Nov 2019 21:49:30 +0100 Message-ID: <7934bc2b-597f-0bb3-be2d-32f3b07b4de9@hartkopp.net> References: <0000000000005c08d10597a3a05d@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: linux-can.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