[PATCH 01/10] net: add net_eth_to_udp() helper for validated UDP extraction

[Ahmad Fatoum]

On 4/2/26 8:36 AM, Sascha Hauer wrote:
> The existing net_eth_to_udp_payload(), net_eth_to_udphdr() and
> net_eth_to_udplen() helpers parse protocol headers by offset arithmetic
> without checking the actual packet length. A short or malformed packet
> can cause out-of-bounds reads.
> 
> Introduce net_eth_to_udp() which takes the frame pointer and the
> NIC-reported length, validates that the packet is large enough to
> contain ethernet + IP + UDP headers, cross-checks the UDP length field
> against available bytes, and returns all three pieces of information
> (udp header, payload pointer, payload length) callers typically need.
> 
> The old helpers are kept for now; callers will be converted in subsequent
> commits.
> 
> Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
> Co-Authored-By: Claude Opus 4.6 <noreply at anthropic.com>

Reviewed-by: Ahmad Fatoum <a.fatoum at pengutronix.de>

> ---
>  include/net.h |  9 +++++++++
>  net/net.c     | 40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/include/net.h b/include/net.h
> index 43d718353a..bdf39bc531 100644
> --- a/include/net.h
> +++ b/include/net.h
> @@ -308,6 +308,15 @@ static inline int net_eth_to_udplen(char *pkt)
>  	return ntohs(udp->uh_ulen) - 8;
>  }
>  
> +struct net_udp_pkt {
> +	struct udphdr *udp;
> +	void *payload;
> +	unsigned int len;
> +};
> +
> +int net_eth_to_udp(char *pkt, unsigned int framelen,
> +		   struct net_udp_pkt *udp_pkt);
> +
>  int net_checksum_ok(unsigned char *, int);	/* Return true if cksum OK	*/
>  uint16_t net_checksum(unsigned char *, int);	/* Calculate the checksum	*/
>  
> diff --git a/net/net.c b/net/net.c
> index fc32c4562b..67c0eeb2ae 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -60,6 +60,46 @@ const char *net_get_domainname(void)
>  	return net_domainname;
>  }
>  
> +/**
> + * net_eth_to_udp - extract and validate UDP payload from an ethernet frame
> + * @pkt:	pointer to start of ethernet frame
> + * @framelen:	total frame length as reported by the NIC
> + * @udp_pkt:	output struct, filled on success
> + *
> + * Validates that the frame is large enough to contain the ethernet, IP and
> + * UDP headers and clamps the reported UDP payload length to what is actually
> + * available in the frame.
> + *
> + * Return: 0 on success, negative error code on malformed/short packets.
> + */
> +int net_eth_to_udp(char *pkt, unsigned int framelen,
> +		   struct net_udp_pkt *udp_pkt)
> +{
> +	unsigned int hdr_len = ETHER_HDR_SIZE + sizeof(struct iphdr) +
> +			       sizeof(struct udphdr);
> +	struct udphdr *udp;
> +	unsigned int payload_len;
> +
> +	if (framelen < hdr_len)
> +		return -EINVAL;
> +
> +	udp = (struct udphdr *)((struct iphdr *)(pkt + ETHER_HDR_SIZE) + 1);
> +
> +	if (ntohs(udp->uh_ulen) < sizeof(struct udphdr))
> +		return -EINVAL;
> +
> +	payload_len = ntohs(udp->uh_ulen) - sizeof(struct udphdr);
> +
> +	if (payload_len > framelen - hdr_len)
> +		return -EINVAL;
> +
> +	udp_pkt->udp = udp;
> +	udp_pkt->payload = (char *)(udp + 1);
> +	udp_pkt->len = payload_len;
> +
> +	return 0;
> +}
> +
>  int net_checksum_ok(unsigned char *ptr, int len)
>  {
>  	return net_checksum(ptr, len) == 0xffff;
> 

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