Re: [PATCH net v3] nfc: nci: Fix uninit-value in nci_rx_work

[Krzysztof Kozlowski]

On 02/05/2024 10:22, Ryosuke Yasuoka wrote:
> syzbot reported the following uninit-value access issue [1]
> 
> nci_rx_work() parses received packet from ndev->rx_q. It should be
> validated header size, payload size and total packet size before
> processing the packet. If an invalid packet is detected, it should be
> silently discarded.
> 
> Fixes: d24b03535e5e ("nfc: nci: Fix uninit-value in nci_dev_up and nci_ntf_packet")
> Reported-and-tested-by: syzbot+d7b4dc6cd50410152534@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=d7b4dc6cd50410152534 [1]
> Signed-off-by: Ryosuke Yasuoka <ryasuoka@xxxxxxxxxx>
> ---
> 
> v3
> - As Simon pointed out, the valid packets will reach invalid_pkt_free
> and kfree_skb(skb) after being handled correctly in switch statement.
> It can lead to double free issues, which is not intended. So this patch
> uses "continue" instead of "break" in switch statement.
> 
> - In the current implementation, once zero payload size is detected, the
> for statement exits. It should continue processing subsequent packets. 
> So this patch just frees skb in invalid_pkt_free when the invalid 
> packets are detected.
> 
> v2
> https://lore.kernel.org/lkml/20240428134525.GW516117@xxxxxxxxxx/T/
> 
> - The v1 patch only checked whether skb->len is zero. This patch also
>   checks header size, payload size and total packet size.
> 
> 
> v1
> https://lore.kernel.org/linux-kernel/CANn89iJrQevxPFLCj2P=U+XSisYD0jqrUQpa=zWMXTjj5+RriA@xxxxxxxxxxxxxx/T/
> 
> 
>  net/nfc/nci/core.c | 33 ++++++++++++++++++++++++---------
>  1 file changed, 24 insertions(+), 9 deletions(-)
> 
> diff --git a/net/nfc/nci/core.c b/net/nfc/nci/core.c
> index 0d26c8ec9993..e4f92a090022 100644
> --- a/net/nfc/nci/core.c
> +++ b/net/nfc/nci/core.c
> @@ -1463,6 +1463,16 @@ int nci_core_ntf_packet(struct nci_dev *ndev, __u16 opcode,
>  				 ndev->ops->n_core_ops);
>  }
>  
> +static bool nci_valid_size(struct sk_buff *skb, unsigned int header_size)
> +{
> +	if (skb->len < header_size ||
> +	    !nci_plen(skb->data) ||
> +	    skb->len < header_size + nci_plen(skb->data)) {
> +		return false;
> +	}
> +	return true;
> +}
> +
>  /* ---- NCI TX Data worker thread ---- */
>  
>  static void nci_tx_work(struct work_struct *work)
> @@ -1516,30 +1526,35 @@ static void nci_rx_work(struct work_struct *work)
>  		nfc_send_to_raw_sock(ndev->nfc_dev, skb,
>  				     RAW_PAYLOAD_NCI, NFC_DIRECTION_RX);
>  
> -		if (!nci_plen(skb->data)) {
> -			kfree_skb(skb);
> -			break;
> -		}
> +		if (!skb->len)
> +			goto invalid_pkt_free;
>  
>  		/* Process frame */
>  		switch (nci_mt(skb->data)) {
>  		case NCI_MT_RSP_PKT:
> +			if (!nci_valid_size(skb, NCI_CTRL_HDR_SIZE))
> +				goto invalid_pkt_free;
>  			nci_rsp_packet(ndev, skb);
> -			break;
> +			continue;

I don't find this code readable.

>  
>  		case NCI_MT_NTF_PKT:
> +			if (!nci_valid_size(skb, NCI_CTRL_HDR_SIZE))
> +				goto invalid_pkt_free;
>  			nci_ntf_packet(ndev, skb);
> -			break;
> +			continue;
>  
>  		case NCI_MT_DATA_PKT:
> +			if (!nci_valid_size(skb, NCI_DATA_HDR_SIZE))
> +				goto invalid_pkt_free;
>  			nci_rx_data_packet(ndev, skb);
> -			break;
> +			continue;
>  
>  		default:
>  			pr_err("unknown MT 0x%x\n", nci_mt(skb->data));
> -			kfree_skb(skb);
> -			break;
> +			goto invalid_pkt_free;
>  		}
> +invalid_pkt_free:
> +		kfree_skb(skb);

Why you cannot kfree in "default" and error cases? I don't think that
goto inside loop makes this code easier to follow.

>  	}
>  
>  	/* check if a data exchange timeout has occurred */

Best regards,
Krzysztof