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 # calls skb_get_hash
tcp dport 42 reject with tcp reset # emits skb with NULL skb dev/sk
}
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 <cpaasch@xxxxxxxxx>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/494
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
include/linux/skbuff.h | 12 ++++++++++--
net/core/flow_dissector.c | 14 ++++++++++----
net/netfilter/nf_tables_trace.c | 2 +-
3 files changed, 21 insertions(+), 7 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index fe7d8dbef77e..6e78019f899a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1498,7 +1498,7 @@ __skb_set_sw_hash(struct sk_buff *skb, __u32 hash, bool is_l4)
__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,
@@ -1578,10 +1578,18 @@ void skb_flow_dissect_hash(const struct sk_buff *skb,
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;
}
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 59fe46077b3c..32454181be60 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1860,7 +1860,7 @@ u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);
/**
- * __skb_get_hash: calculate a flow hash
+ * __skb_get_hash_net: calculate a flow hash
* @skb: sk_buff to calculate flow hash from
*
* This function calculates a flow hash based on src/dst addresses
@@ -1868,18 +1868,24 @@ EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);
* 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)
diff --git a/net/netfilter/nf_tables_trace.c b/net/netfilter/nf_tables_trace.c
index a83637e3f455..580c55268f65 100644
--- a/net/netfilter/nf_tables_trace.c
+++ b/net/netfilter/nf_tables_trace.c
@@ -317,7 +317,7 @@ void nft_trace_init(struct nft_traceinfo *info, const struct nft_pktinfo *pkt,
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);
}
--
2.44.2
