net: add and use skb_get_hash_net

Years ago flow dissector gained ability to delegate flow dissection
to a bpf program, scoped per netns.

Unfortunately, skb_get_hash() only gets an sk_buff argument instead
of both net+skb. This means the flow dissector needs to obtain the
netns pointer from somewhere else.

The netns is derived from skb->dev, and if that is not available, from
skb->sk. If neither is set, we hit a (benign) WARN_ON_ONCE().

Trying both dev and sk covers most cases, but not all, as recently
reported by Christoph Paasch.

In case of nf-generated tcp reset, both sk and dev are NULL:

WARNING: .. net/core/flow_dissector.c:1104
skb_flow_dissect_flow_keys include/linux/skbuff.h:1536 [inline]
skb_get_hash include/linux/skbuff.h:1578 [inline]
nft_trace_init+0x7d/0x120 net/netfilter/nf_tables_trace.c:320
nft_do_chain+0xb26/0xb90 net/netfilter/nf_tables_core.c:268
nft_do_chain_ipv4+0x7a/0xa0 net/netfilter/nft_chain_filter.c:23
nf_hook_slow+0x57/0x160 net/netfilter/core.c:626
__ip_local_out+0x21d/0x260 net/ipv4/ip_output.c:118
ip_local_out+0x26/0x1e0 net/ipv4/ip_output.c:127
nf_send_reset+0x58c/0x700 net/ipv4/netfilter/nf_reject_ipv4.c:308
nft_reject_ipv4_eval+0x53/0x90 net/ipv4/netfilter/nft_reject_ipv4.c:30
[..]

syzkaller did something like this:
table inet filter {
chain input {
type filter hook input priority filter; policy accept;
meta nftrace set 1
tcp dport 42 reject with tcp reset
}
chain output {
type filter hook output priority filter; policy accept;
# empty chain is enough
}
}

... then sends a tcp packet to port 42.

Initial attempt to simply set skb->dev from nf_reject_ipv4 doesn't cover
all cases: skbs generated via ipv4 igmp_send_report trigger similar splat.

Moreover, Pablo Neira found that nft_hash.c uses __skb_get_hash_symmetric()
which would trigger same warn splat for such skbs.

Lets allow callers to pass the current netns explicitly.
The nf_trace infrastructure is adjusted to use the new helper.

__skb_get_hash_symmetric is handled in the next patch.

Reported-by: Christoph Paasch <[e-mail]>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/494
Reviewed-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/skbuff.h

···
 	__skb_set_hash(skb, hash, true, is_l4);
 }
 
-void __skb_get_hash(struct sk_buff *skb);
+void __skb_get_hash_net(const struct net *net, struct sk_buff *skb);
 u32 __skb_get_hash_symmetric(const struct sk_buff *skb);
 u32 skb_get_poff(const struct sk_buff *skb);
 u32 __skb_get_poff(const struct sk_buff *skb, const void *data,
···
 			   struct flow_dissector *flow_dissector,
 			   void *target_container);
 
+static inline __u32 skb_get_hash_net(const struct net *net, struct sk_buff *skb)
+{
+	if (!skb->l4_hash && !skb->sw_hash)
+		__skb_get_hash_net(net, skb);
+
+	return skb->hash;
+}
+
 static inline __u32 skb_get_hash(struct sk_buff *skb)
 {
 	if (!skb->l4_hash && !skb->sw_hash)
-		__skb_get_hash(skb);
+		__skb_get_hash_net(NULL, skb);
 
 	return skb->hash;
 }

net/core/flow_dissector.c

···
 EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);
 
 /**
- * __skb_get_hash: calculate a flow hash
+ * __skb_get_hash_net: calculate a flow hash
+ * @net: associated network namespace, derived from @skb if NULL
  * @skb: sk_buff to calculate flow hash from
  *
  * This function calculates a flow hash based on src/dst addresses
···
  * on success, zero indicates no valid hash.  Also, sets l4_hash in skb
  * if hash is a canonical 4-tuple hash over transport ports.
  */
-void __skb_get_hash(struct sk_buff *skb)
+void __skb_get_hash_net(const struct net *net, struct sk_buff *skb)
 {
 	struct flow_keys keys;
 	u32 hash;
 
+	memset(&keys, 0, sizeof(keys));
+
+	__skb_flow_dissect(net, skb, &flow_keys_dissector,
+			   &keys, NULL, 0, 0, 0,
+			   FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
+
 	__flow_hash_secret_init();
 
-	hash = ___skb_get_hash(skb, &keys, &hashrnd);
+	hash = __flow_hash_from_keys(&keys, &hashrnd);
 
 	__skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
 }
-EXPORT_SYMBOL(__skb_get_hash);
+EXPORT_SYMBOL(__skb_get_hash_net);
 
 __u32 skb_get_hash_perturb(const struct sk_buff *skb,
 			   const siphash_key_t *perturb)

net/netfilter/nf_tables_trace.c

···
 	net_get_random_once(&trace_key, sizeof(trace_key));
 
 	info->skbid = (u32)siphash_3u32(hash32_ptr(skb),
-					skb_get_hash(skb),
+					skb_get_hash_net(nft_net(pkt), skb),
 					skb->skb_iif,
 					&trace_key);
 }