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([2620:15c:2c1:200:55c7:81e6:c7d8:94b]) by smtp.gmail.com with ESMTPSA id y24sm28215358pfr.116.2019.11.19.13.09.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Nov 2019 13:09:17 -0800 (PST) Subject: Re: KMSAN: uninit-value in can_receive To: Oliver Hartkopp , Eric Dumazet , 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> <7934bc2b-597f-0bb3-be2d-32f3b07b4de9@hartkopp.net> <7f5c4546-0c1a-86ae-581e-0203b5fca446@pengutronix.de> <1f7d6ea7-152e-ff18-549c-b196d8b5e3a7@hartkopp.net> <9e06266a-67f3-7352-7b87-2b9144c7c9a9@gmail.com> <3142c032-e46a-531c-d1b8-d532e5b403a6@hartkopp.net> From: Eric Dumazet Message-ID: <92c04159-b83a-3e33-91da-25a727a692d0@gmail.com> Date: Tue, 19 Nov 2019 13:09:16 -0800 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: <3142c032-e46a-531c-d1b8-d532e5b403a6@hartkopp.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 11/19/19 12:24 PM, Oliver Hartkopp wrote: > Hi Eric, > > On 19/11/2019 17.53, Eric Dumazet wrote: >> >> >> On 11/18/19 11:35 PM, Oliver Hartkopp wrote: >>> >> >>> >>> See ioctl$ifreq https://syzkaller.appspot.com/x/log.txt?x=14563416e00000 >>> >>> 23:11:34 executing program 2: >>> r0 = socket(0x200000000000011, 0x3, 0x0) >>> ioctl$ifreq_SIOCGIFINDEX_vcan(r0, 0x8933, &(0x7f0000000040)={'vxcan1\x00', 0x0}) >>> bind$packet(r0, &(0x7f0000000300)={0x11, 0xc, r1}, 0x14) >>> sendmmsg(r0, &(0x7f0000000d00), 0x400004e, 0x0) >>> >>> We only can receive skbs from (v(x))can devices. >>> No matter if someone wrote to them via PF_CAN or PF_PACKET. >>> We check for ETH_P_CAN(FD) type and ARPHRD_CAN dev type at rx time. >> >> And what entity sets the can_skb_prv(skb)->skbcnt to zero exactly ? >> >>> >>>>> 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? >>> >>> So this question remains still valid whether we have a false positive from KMSAN here. >> >> I do not believe it is a false positive. >> >> It seems CAN relies on some properties of low level drivers using alloc_can_skb() or similar function. >> >> Why not simply fix this like that ? >> >> diff --git a/net/can/af_can.c b/net/can/af_can.c >> index 128d37a4c2e0ba5d8db69fcceec8cbd6a79380df..3e71a78d82af84caaacd0ef512b5e894efbf4852 100644 >> --- a/net/can/af_can.c >> +++ b/net/can/af_can.c >> @@ -647,8 +647,9 @@ static void can_receive(struct sk_buff *skb, struct net_device *dev) >>          pkg_stats->rx_frames_delta++; >>            /* create non-zero unique skb identifier together with *skb */ >> -       while (!(can_skb_prv(skb)->skbcnt)) >> +       do { >>                  can_skb_prv(skb)->skbcnt = atomic_inc_return(&skbcounter); >> +       } while (!(can_skb_prv(skb)->skbcnt)); >>            rcu_read_lock(); >>   > > Please check commit d3b58c47d330d ("can: replace timestamp as unique skb attribute"). Oh well... This notion of 'unique skb attribute' is interesting... > > can_skb_prv(skb)->skbcnt is set to 0 at skb creation time when sending CAN frames from local host or receiving CAN frames from a real CAN interface. We can not enforce this to happen with a virtual interface. > > When a CAN skb is received by the net layer the *first* time it gets a unique value which we need for a per-cpu filter mechanism in raw_rcv(). > > So where's the problem to check for (!(can_skb_prv(skb)->skbcnt)) in a while statement? I can't see a chance for an uninitialized value there. You have to make sure the packet has been properly cooked by a 'real CAN interface' then, and reject them if not.