Re: [PATCHv5 net 2/2] IPv6: reply ICMP error if the first fragment don't include all headers

["Georg Kohmann (geokohma)" <geokohma@cisco.com>]

On 27.10.2020 03:28, Hangbin Liu wrote:
> Based on RFC 8200, Section 4.5 Fragment Header:
>
>   -  If the first fragment does not include all headers through an
>      Upper-Layer header, then that fragment should be discarded and
>      an ICMP Parameter Problem, Code 3, message should be sent to
>      the source of the fragment, with the Pointer field set to zero.
>
> As the packet may be any kind of L4 protocol, I only checked some common
> protocols' header length and handle others by (offset + 1) > skb->len.
> Checking each packet header in IPv6 fast path will have performance impact,
> so I put the checking in ipv6_frag_rcv().
>
> When send ICMP error message, if the 1st truncated fragment is ICMP message,
> icmp6_send() will break as is_ineligible() return true. So I added a check
> in is_ineligible() to let fragment packet with nexthdr ICMP but no ICMP header
> return false.
>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> ---
> v5:
> Only check nexthdr if ipv6_skip_exthdr() does not return -1. For
> IPPROTO_NONE/NEXTHDR_NONE, later code will handle and ignore it.
>
> v4:
> remove unused variable
>
> v3:
> a) use frag_off to check if this is a fragment packet
> b) check some common protocols' header length
>
> v2:
> a) Move header check to ipv6_frag_rcv(). Also check the ipv6_skip_exthdr()
>    return value
> b) Fix ipv6_find_hdr() parameter type miss match in is_ineligible()
>
> ---
>  net/ipv6/icmp.c       |  8 +++++++-
>  net/ipv6/reassembly.c | 33 ++++++++++++++++++++++++++++++++-
>  2 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
> index ec448b71bf9a..8956144ea65e 100644
> --- a/net/ipv6/icmp.c
> +++ b/net/ipv6/icmp.c
> @@ -158,7 +158,13 @@ static bool is_ineligible(const struct sk_buff *skb)
>  		tp = skb_header_pointer(skb,
>  			ptr+offsetof(struct icmp6hdr, icmp6_type),
>  			sizeof(_type), &_type);
> -		if (!tp || !(*tp & ICMPV6_INFOMSG_MASK))
> +
> +		/* Based on RFC 8200, Section 4.5 Fragment Header, return
> +		 * false if this is a fragment packet with no icmp header info.
> +		 */
> +		if (!tp && frag_off != 0)
> +			return false;
> +		else if (!tp || !(*tp & ICMPV6_INFOMSG_MASK))
>  			return true;
>  	}
>  	return false;
> diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
> index 1f5d4d196dcc..effe1d086e5d 100644
> --- a/net/ipv6/reassembly.c
> +++ b/net/ipv6/reassembly.c
> @@ -42,6 +42,8 @@
>  #include <linux/skbuff.h>
>  #include <linux/slab.h>
>  #include <linux/export.h>
> +#include <linux/tcp.h>
> +#include <linux/udp.h>
>  
>  #include <net/sock.h>
>  #include <net/snmp.h>
> @@ -322,7 +324,9 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
>  	struct frag_queue *fq;
>  	const struct ipv6hdr *hdr = ipv6_hdr(skb);
>  	struct net *net = dev_net(skb_dst(skb)->dev);
> -	int iif;
> +	__be16 frag_off;
> +	int iif, offset;
> +	u8 nexthdr;
>  
>  	if (IP6CB(skb)->flags & IP6SKB_FRAGMENTED)
>  		goto fail_hdr;
> @@ -351,6 +355,33 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
>  		return 1;
>  	}
>  
> +	/* RFC 8200, Section 4.5 Fragment Header:
> +	 * If the first fragment does not include all headers through an
> +	 * Upper-Layer header, then that fragment should be discarded and
> +	 * an ICMP Parameter Problem, Code 3, message should be sent to
> +	 * the source of the fragment, with the Pointer field set to zero.
> +	 */
> +	nexthdr = hdr->nexthdr;
> +	offset = ipv6_skip_exthdr(skb, skb_transport_offset(skb), &nexthdr, &frag_off);
> +	if (offset >= 0) {
> +		/* Check some common protocols' header */
> +		if (nexthdr == IPPROTO_TCP)
> +			offset += sizeof(struct tcphdr);
> +		else if (nexthdr == IPPROTO_UDP)
> +			offset += sizeof(struct udphdr);
> +		else if (nexthdr == IPPROTO_ICMPV6)
> +			offset += sizeof(struct icmp6hdr);
> +		else
> +			offset += 1;
> +
> +		if (frag_off == htons(IP6_MF) && offset > skb->len) {

This do not catch atomic fragments (fragmented packet with only one fragment). frag_off also contains two reserved bits (both 0) that might change in the future. I suggest you only check that the offset is 0:

frag_off & htons(IP6_OFFSET)

Sorry for not commenting on this earlier.

> +			__IP6_INC_STATS(net, __in6_dev_get_safely(skb->dev),
> +					IPSTATS_MIB_INHDRERRORS);
> +			icmpv6_param_prob(skb, ICMPV6_HDR_INCOMP, 0);
> +			return -1;
> +		}
> +	}
> +
>  	iif = skb->dev ? skb->dev->ifindex : 0;
>  	fq = fq_find(net, fhdr->identification, hdr, iif);
>  	if (fq) {