Re: [PATCH bpf-next v10 1/5] bpf: add bpf_icmp_send kfunc

[Daniel Borkmann <daniel@iogearbox.net>]

On 6/29/26 12:35 PM, Mahe Tardy wrote:
> On Fri, Jun 26, 2026 at 09:18:39AM -0700, Stanislav Fomichev wrote:
>> On 06/25, Mahe Tardy wrote:
>>> On Thu, Jun 25, 2026 at 09:24:59AM -0700, Stanislav Fomichev wrote:
>>>> On 06/25, Mahe Tardy wrote:
>>>
>>> [...]
>>>
>>>>> +__bpf_kfunc int bpf_icmp_send(struct __sk_buff *skb_ctx, int type, int code)
>>>>> +{
>>>>> +	struct sk_buff *skb = (struct sk_buff *)skb_ctx;
>>>>> +	struct sk_buff *nskb;
>>>>> +	struct sock *sk;
>>>>> +
>>>>> +	sk = skb_to_full_sk(skb);
>>>>> +	if (sk && sk->sk_kern_sock &&
>>>>> +	    (sk->sk_protocol == IPPROTO_ICMP || sk->sk_protocol == IPPROTO_ICMPV6))
>>>>> +		return -EBUSY;
>>>>> +
>>>>> +	switch (skb->protocol) {
>>>>> +#if IS_ENABLED(CONFIG_INET)
>>>>> +	case htons(ETH_P_IP): {
>>>>> +		if (type != ICMP_DEST_UNREACH)
>>>>> +			return -EOPNOTSUPP;
>>>>> +		if (code < 0 || code > NR_ICMP_UNREACH ||
>>>>> +		    code == ICMP_FRAG_NEEDED) /* needs a valid next-hop MTU */
>>>>> +			return -EINVAL;
>>>>> +
>>>>> +		/* icmp_send expects skb_dst to be a real rtable. */
>>>>> +		if (!skb_valid_dst(skb))
>>>>> +			return -ENETUNREACH;
>>>>> +
>>>>> +		nskb = skb_clone(skb, GFP_ATOMIC);
>>>>> +		if (!nskb)
>>>>> +			return -ENOMEM;
>>>>> +
>>>>> +		memset(IPCB(nskb), 0, sizeof(*IPCB(nskb)));
>>>>> +		icmp_send(nskb, type, code, 0);
>>>>> +		consume_skb(nskb);
>>>>> +		break;
>>>>> +	}
>>>>> +#endif
>>>>> +#if IS_ENABLED(CONFIG_IPV6)
>>>>> +	case htons(ETH_P_IPV6):
>>>>> +		if (type != ICMPV6_DEST_UNREACH)
>>>>> +			return -EOPNOTSUPP;
>>>>> +		if (code < 0 || code > ICMPV6_REJECT_ROUTE)
>>>>> +			return -EINVAL;
>>>>
>>>> [..]
>>>>
>>>>> +		/* icmpv6_send may treat skb_dst as rt6_info. */
>>>>> +		if (skb_metadata_dst(skb))
>>>>> +			return -ENETUNREACH;
>>>>
>>>> A bit confused about this. Which part of icmpv6_send treats skb_dst as rt6_info?
>>>> (I see the original sashiko report about dst, but icmp6 seems to be not
>>>> requiring it)
>>>
>>> Yeah I was also a bit confused because this came out of nowhere as soon
>>> as I put the skb_valid_dst only on the IPv4 path (for different
>>> reasons), but there is actually a potential trace in which we have type
>>> confusion indeed:
>>>
>>> - icmp6_send() checks scoped source addresses and calls icmp6_iif() at net/ipv6/icmp.c:702
>>> - icmp6_iif() calls icmp6_dev() at net/ipv6/icmp.c:441
>>> - icmp6_dev() does skb_rt6_info(skb) for loopback/L3 master devices at net/ipv6/icmp.c:428
>>> - skb_rt6_info() casts any non-NULL dst to struct rt6_info at include/net/ip6_route.h:233
>>> - rt6->rt6i_idev is then dereferenced at net/ipv6/icmp.c:434
>>>
>>> When checking with pahole, we can find this on my local kernel:
>>>
>>> struct rt6_info {
>>> 	struct dst_entry           dst;                  /*     0   136 */
>>> 	/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
>>> 	struct fib6_info *         from;                 /*   136     8 */
>>> 	int                        sernum;               /*   144     4 */
>>> 	struct rt6key              rt6i_dst;             /*   148    20 */
>>> 	struct rt6key              rt6i_src;             /*   168    20 */
>>> 	struct in6_addr            rt6i_gateway;         /*   188    16 */
>>>
>>> 	/* XXX 4 bytes hole, try to pack */
>>>
>>> 	/* --- cacheline 3 boundary (192 bytes) was 16 bytes ago --- */
>>> 	struct inet6_dev *         rt6i_idev;            /*   208     8 */  <--- we dereference this
>>> 	u32                        rt6i_flags;           /*   216     4 */
>>> 	short unsigned int         rt6i_nfheader_len;    /*   220     2 */
>>>
>>> 	/* size: 224, cachelines: 4, members: 9 */
>>> 	/* sum members: 218, holes: 1, sum holes: 4 */
>>> 	/* padding: 2 */
>>> 	/* last cacheline: 32 bytes */
>>> };
>>>
>>> And the metadata_dst would look like this:
>>>
>>> struct metadata_dst {
>>> 	struct dst_entry           dst;                  /*     0   136 */
>>> 	/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
>>> 	enum metadata_type         type;                 /*   136     4 */
>>>
>>> 	/* XXX 4 bytes hole, try to pack */
>>>
>>> 	union {
>>> 		struct ip_tunnel_info tun_info;          /*   144    96 */
>>> 		struct hw_port_info port_info;           /*   144    16 */
>>> 		struct macsec_info macsec_info;          /*   144     8 */
>>> 		struct xfrm_md_info xfrm_info;           /*   144    16 */
>>> 	} u;                                             /*   144    96 */  <--- we land on this union
>>>
>>> 	/* size: 240, cachelines: 4, members: 3 */
>>> 	/* sum members: 236, holes: 1, sum holes: 4 */
>>> 	/* last cacheline: 48 bytes */
>>> };
>>>
>>> Let's say it's a struct ip_tunnel_info:
>>>
>>> struct ip_tunnel_info {
>>> 	struct ip_tunnel_key       key;                  /*     0    64 */
>>>
>>> 	/* XXX last struct has 7 bytes of padding */
>>>
>>> 	/* --- cacheline 1 boundary (64 bytes) --- */
>>> 	struct ip_tunnel_encap     encap;                /*    64     8 */  <--- 144 + 64 = 208 we land here
>>> 	struct dst_cache           dst_cache;            /*    72    16 */
>>> 	u8                         options_len;          /*    88     1 */
>>> 	u8                         mode;                 /*    89     1 */
>>>
>>> 	/* size: 96, cachelines: 2, members: 5 */
>>> 	/* padding: 6 */
>>> 	/* paddings: 1, sum paddings: 7 */
>>> 	/* last cacheline: 32 bytes */
>>> };
>>>
>>> So I imagine this is fairly tricky to trigger but still a case of type
>>> confusion. I have actually no idea how likely this can happen from my
>>> call but the trace makes sense at least.
>>
>> That logic seems to exist for the icmp6_send to find the input device
>> (since the expected use-case for calling icmp6_send is to the incoming
>> skb). And since you're mainly doing egress, I don't think this path will
>> ever trigger (iow the check is not needed)?
>>
>> Maybe you can add cgroup_ingress test case? Looks like this rt6_info
>> path might trigger for ipv6 lo? I don't see any ingress test in your
>> series, so might be good to have one regardless?
> 
> The initial reason I added only egress is because the use case of this
> makes more sense if that's your local kernel giving you feedback about a
> connection you are trying to establish, as a process, but is prevented.
> 
> But indeed, I could extend the test to ingress as well, I'd just like
> ideally getting an ack from networking maintainers since this is already
> v10 of this, before making some new changes.

Sry for the delay, Mahe. Not speaking for net maintainers, but if the
skb_metadata_dst() test in IPv6 would be made more strict and align with
IPv4 by erroring out on !skb_valid_dst(skb), would this work? It would
reject NULL dst for IPv6 case when someone would push a synthetic TEST_RUN
skb to spoofe an icmpv6 injection without CAP_NET_ADMIN fwiw. Otherwise
lgtm.

Thanks,
Daniel