Subject: Re: [PATCH net-next 02/12] gtp: properly parse extension headers

From: Simon Horman <[email-masked]>
Date: Fri, 26 Apr 2024 21:28:52 +0100

On Thu, Apr 25, 2024 at 12:51:28PM +0200, Pablo Neira Ayuso wrote:
> Currently GTP packets are dropped if the next extension field is set to
> non-zero value, but this are valid GTP packets.
> 
> TS 29.281 provides a longer header format, which is defined as struct
> gtp1_header_long. Such long header format is used if any of the S, PN, E
> flags is set.
> 
> This long header is 4 bytes longer than struct gtp1_header, plus
> variable length (optional) extension headers. The next extension header
> field is zero is no extension header is provided.
> 
> The extension header is composed of a length field which includes total
> number of 4 byte words including the extension header itself (1 byte),
> payload (variable length) and next type (1 byte). The extension header
> size and its payload is aligned to 4 bytes.
> 
> A GTP packet might come with a chain extensions headers, which makes it
> slightly cumbersome to parse because the extension next header field
> comes at the end of the extension header, and there is a need to check
> if this field becomes zero to stop the extension header parser.
> 
> Signed-off-by: Pablo Neira Ayuso <[email-masked]>
> ---
>  drivers/net/gtp.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  include/net/gtp.h |  5 +++++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
> index 4680cdf4fa70..9451c74c1a7d 100644
> --- a/drivers/net/gtp.c
> +++ b/drivers/net/gtp.c
> @@ -567,6 +567,43 @@ static int gtp1u_handle_echo_resp(struct gtp_dev *gtp, struct sk_buff *skb)
>  				       msg, 0, GTP_GENL_MCGRP, GFP_ATOMIC);
>  }
>  
> +static int gtp_parse_exthdrs(struct sk_buff *skb, unsigned int *hdrlen)
> +{
> +	struct gtp_ext_hdr *gtp_exthdr, _gtp_exthdr;
> +	unsigned int offset = *hdrlen;
> +	__u8 *next_type, _next_type;
> +
> +	/* From 29.060: "The Extension Header Length field specifies the length
> +	 * of the particular Extension header in 4 octets units."
> +	 *
> +	 * This length field includes length field size itself (1 byte),
> +	 * payload (variable length) and next type (1 byte). The extension
> +	 * header is aligned to to 4 bytes.
> +	 */
> +
> +	do {
> +		gtp_exthdr = skb_header_pointer(skb, offset, sizeof(gtp_exthdr),

Hi Pablo,

Should this be sizeof(*gtp_exthdr)?

And likewise, in the ip_version calculation in gtp_inner_proto()
in [PATCH 11/12] gtp: support for IPv4-in-IPv6-GTP and IPv6-in-IPv4-GTP 

Flagged by Coccinelle.

> +						&_gtp_exthdr);
> +		if (!gtp_exthdr || !gtp_exthdr->len)
> +			return -1;
> +
> +		offset += gtp_exthdr->len * 4;
> +
> +		/* From 29.060: "If no such Header follows, then the value of
> +		 * the Next Extension Header Type shall be 0."
> +		 */
> +		next_type = skb_header_pointer(skb, offset - 1,
> +					       sizeof(_next_type), &_next_type);
> +		if (!next_type)
> +			return -1;
> +
> +	} while (*next_type != 0);
> +
> +	*hdrlen = offset;
> +
> +	return 0;
> +}

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