Re: [PATCH net] can: isotp: isotp_rcv_cf(): fix so->rx race problem

[Oliver Hartkopp]

Hi,

the referenced syzbot issue has already been fixed in upstream here:

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=5f33a09e769a9da0482f20a6770a342842443776

("can: isotp: convert struct tpcon::{idx,len} to unsigned int")

Additionally this fix changes some behaviour that is required by the ISO 
16765-2 specification (see below).

On 17.01.22 13:01, Ziyang Xuan wrote:
> When receive a FF, the current code logic does not consider the real
> so->rx.state but set so->rx.state to ISOTP_IDLE directly. That will
> make so->rx accessed by multiple receiving processes concurrently.

This is intentionally. "multiple receiving processes" are not allowed 
resp. specified by ISO 15765-2.

> The following syz problem is one of the scenarios. so->rx.len is
> changed by isotp_rcv_ff() during isotp_rcv_cf(), so->rx.len equals
> 0 before alloc_skb() and equals 4096 after alloc_skb(). That will
> trigger skb_over_panic() in skb_put().
>
> =======================================================
> CPU: 1 PID: 19 Comm: ksoftirqd/1 Not tainted 5.16.0-rc8-syzkaller #0
> RIP: 0010:skb_panic+0x16c/0x16e net/core/skbuff.c:113
> Call Trace:
>   <TASK>
>   skb_over_panic net/core/skbuff.c:118 [inline]
>   skb_put.cold+0x24/0x24 net/core/skbuff.c:1990
>   isotp_rcv_cf net/can/isotp.c:570 [inline]
>   isotp_rcv+0xa38/0x1e30 net/can/isotp.c:668
>   deliver net/can/af_can.c:574 [inline]
>   can_rcv_filter+0x445/0x8d0 net/can/af_can.c:635
>   can_receive+0x31d/0x580 net/can/af_can.c:665
>   can_rcv+0x120/0x1c0 net/can/af_can.c:696
>   __netif_receive_skb_one_core+0x114/0x180 net/core/dev.c:5465
>   __netif_receive_skb+0x24/0x1b0 net/core/dev.c:5579
>
> Check so->rx.state equals ISOTP_IDLE firstly in isotp_rcv_ff().
> Make sure so->rx idle when receive another new packet. And set
> so->rx.state to ISOTP_IDLE after whole packet being received.
>
> Fixes: e057dd3fc20f ("can: add ISO 15765-2:2016 transport protocol")
> Reported-by: syzbot+4c63f36709a642f801c5@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
> ---
>   net/can/isotp.c | 28 +++++++++++++++++-----------
>   1 file changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/net/can/isotp.c b/net/can/isotp.c
> index df6968b28bf4..a4b174f860f3 100644
> --- a/net/can/isotp.c
> +++ b/net/can/isotp.c
> @@ -443,8 +443,10 @@ static int isotp_rcv_ff(struct sock *sk, struct canfd_frame *cf, int ae)
>   	int off;
>   	int ff_pci_sz;
>   
> +	if (so->rx.state != ISOTP_IDLE)
> +		return 0;
> +
>   	hrtimer_cancel(&so->rxtimer);
> -	so->rx.state = ISOTP_IDLE;

No matter in which receiving state we receive a first frame (FF) we are 
required to start a fresh reception process and/or terminate the current 
attempt.

Best regards,
Oliver

>   
>   	/* get the used sender LL_DL from the (first) CAN frame data length */
>   	so->rx.ll_dl = padlen(cf->len);
> @@ -518,8 +520,6 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>   		so->lastrxcf_tstamp = skb->tstamp;
>   	}
>   
> -	hrtimer_cancel(&so->rxtimer);
> -
>   	/* CFs are never longer than the FF */
>   	if (cf->len > so->rx.ll_dl)
>   		return 1;
> @@ -531,15 +531,15 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>   			return 1;
>   	}
>   
> +	hrtimer_cancel(&so->rxtimer);
> +
>   	if ((cf->data[ae] & 0x0F) != so->rx.sn) {
>   		/* wrong sn detected - report 'illegal byte sequence' */
>   		sk->sk_err = EILSEQ;
>   		if (!sock_flag(sk, SOCK_DEAD))
>   			sk_error_report(sk);
>   
> -		/* reset rx state */
> -		so->rx.state = ISOTP_IDLE;
> -		return 1;
> +		goto err_out;
>   	}
>   	so->rx.sn++;
>   	so->rx.sn %= 16;
> @@ -551,21 +551,18 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>   	}
>   
>   	if (so->rx.idx >= so->rx.len) {
> -		/* we are done */
> -		so->rx.state = ISOTP_IDLE;
> -
>   		if ((so->opt.flags & ISOTP_CHECK_PADDING) &&
>   		    check_pad(so, cf, i + 1, so->opt.rxpad_content)) {
>   			/* malformed PDU - report 'not a data message' */
>   			sk->sk_err = EBADMSG;
>   			if (!sock_flag(sk, SOCK_DEAD))
>   				sk_error_report(sk);
> -			return 1;
> +			goto err_out;
>   		}
>   
>   		nskb = alloc_skb(so->rx.len, gfp_any());
>   		if (!nskb)
> -			return 1;
> +			goto err_out;
>   
>   		memcpy(skb_put(nskb, so->rx.len), so->rx.buf,
>   		       so->rx.len);
> @@ -573,6 +570,10 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>   		nskb->tstamp = skb->tstamp;
>   		nskb->dev = skb->dev;
>   		isotp_rcv_skb(nskb, sk);
> +
> +		/* we are done */
> +		so->rx.state = ISOTP_IDLE;
> +
>   		return 0;
>   	}
>   
> @@ -591,6 +592,11 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>   	/* we reached the specified blocksize so->rxfc.bs */
>   	isotp_send_fc(sk, ae, ISOTP_FC_CTS);
>   	return 0;
> +
> +err_out:
> +	/* reset rx state */
> +	so->rx.state = ISOTP_IDLE;
> +	return 1;
>   }
>   
>   static void isotp_rcv(struct sk_buff *skb, void *data)