Re: KMSAN: uninit-value in can_receive

From: Eric Dumazet
Date: Tue Nov 19 2019 - 11:53:38 EST

On 11/18/19 11:35 PM, Oliver Hartkopp wrote:

> See ioctl$ifreq
> 23:11:34 executing program 2:
> r0 = socket(0x200000000000011, 0x3, 0x0)
> ioctl$ifreq_SIOCGIFINDEX_vcan(r0, 0x8933, &(0x7f0000000040)={'vxcan1\x00', <r1=>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)

/* 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));