On 05.01.22 12:44, Marc Kleine-Budde wrote:

On 19.12.2021 16:19:20, syzbot wrote:

quoted

  skb_over_panic net/core/skbuff.c:118 [inline]
  skb_over_panic net/core/skbuff.c:118 [inline] net/core/skbuff.c:1986
  skb_put.cold+0x24/0x24 net/core/skbuff.c:1986 net/core/skbuff.c:1986
  isotp_rcv_cf net/can/isotp.c:570 [inline]
  isotp_rcv_cf net/can/isotp.c:570 [inline] net/can/isotp.c:668
  isotp_rcv+0xa38/0x1e30 net/can/isotp.c:668 net/can/isotp.c:668

quoted

struct tpcon {
	int idx;
	int len;

         ^^^

quoted

	u32 state;
	u8 bs;
	u8 sn;
	u8 ll_dl;
	u8 buf[MAX_MSG_LENGTH + 1];
};

static int isotp_rcv_ff(struct sock *sk, struct canfd_frame *cf, int ae)
{

[...]

quoted

	/* Check for FF_DL escape sequence supporting 32 bit PDU length */
	if (so->rx.len) {
		ff_pci_sz = FF_PCI_SZ12;
	} else {
		/* FF_DL = 0 => get real length from next 4 bytes */
		so->rx.len = cf->data[ae + 2] << 24;
		so->rx.len += cf->data[ae + 3] << 16;
		so->rx.len += cf->data[ae + 4] << 8;
		so->rx.len += cf->data[ae + 5];
		ff_pci_sz = FF_PCI_SZ32;
	}

Full 32 Bit PDUs don't work with struct tpcon::len being an "int". I
think converting it to "unsigned int" should be done.

[...]

quoted

}

static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
			struct sk_buff *skb)
{
	struct isotp_sock *so = isotp_sk(sk);
	struct sk_buff *nskb;
	int i;

	if (so->rx.state != ISOTP_WAIT_DATA)
		return 0;

	/* drop if timestamp gap is less than force_rx_stmin nano secs */
	if (so->opt.flags & CAN_ISOTP_FORCE_RXSTMIN) {
		if (ktime_to_ns(ktime_sub(skb->tstamp, so->lastrxcf_tstamp)) <
		    so->force_rx_stmin)
			return 0;

		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;

	/* CFs have usually the LL_DL length */
	if (cf->len < so->rx.ll_dl) {
		/* this is only allowed for the last CF */
		if (so->rx.len - so->rx.idx > so->rx.ll_dl - ae - N_PCI_SZ)
			return 1;
	}

	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;
	}
	so->rx.sn++;
	so->rx.sn %= 16;

	for (i = ae + N_PCI_SZ; i < cf->len; i++) {
		so->rx.buf[so->rx.idx++] = cf->data[i];
		if (so->rx.idx >= so->rx.len)
			break;
	}

	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;
		}

		nskb = alloc_skb(so->rx.len, gfp_any());
		if (!nskb)
			return 1;

		memcpy(skb_put(nskb, so->rx.len), so->rx.buf,

                        ^^^^^^^

quoted

		       so->rx.len);

This is where the skb_over_panic() happens.

Thanks Marc!

Yes I went to this piece of code too - but was not able to find anything 
wrong, as the values at this point should be far(!!) away from INT_MAX.

Due to this check in isotp_rcv_ff():

if (so->rx.len > MAX_MSG_LENGTH) { ... exit

And MAX_MSG_LENGTH is define as 8200.

Btw. making tpcon:len an unsigned int is really the solution to this! 
Which makes the above if-statement act correctly also with values like 
0x80001234.

m(

Thanks for the finding!

Best regards,
Oliver