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netfilter: make nftables drops visible in net dropmonitor

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net_dropmonitor blames core.c:nf_hook_slow.
Add NF_DROP_REASON() helper and use it in nft_do_chain().

The helper releases the skb, so exact drop location becomes
available. Calling code will observe the NF_STOLEN verdict
instead.

Adjust nf_hook_slow so we can embed an erro value wih
NF_STOLEN verdicts, just like we do for NF_DROP.

After this, drop in nftables can be pinpointed to a drop due
to a rule or the chain policy.

Signed-off-by: Florian Westphal <fw@strlen.de>
This commit is contained in:
Florian Westphal 2023-10-11 09:59:38 +02:00
parent 35c038b0a4
commit e0d4593140
3 changed files with 18 additions and 4 deletions
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10
include/linux/netfilter.h
View file

@ -22,6 +22,16 @@ static inline int NF_DROP_GETERR(int verdict)
return -(verdict >> NF_VERDICT_QBITS);
}
static __always_inline int
NF_DROP_REASON(struct sk_buff *skb, enum skb_drop_reason reason, u32 err)
{
BUILD_BUG_ON(err > 0xffff);
kfree_skb_reason(skb, reason);
return ((err << 16) | NF_STOLEN);
}
static inline int nf_inet_addr_cmp(const union nf_inet_addr *a1,
const union nf_inet_addr *a2)
{

6
net/netfilter/core.c
View file

@ -639,10 +639,10 @@ int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state,
if (ret == 1)
continue;
return ret;
case NF_STOLEN:
return NF_DROP_GETERR(verdict);
default:
/* Implicit handling for NF_STOLEN, as well as any other
* non conventional verdicts.
*/
WARN_ON_ONCE(1);
return 0;
}
}

6
net/netfilter/nf_tables_core.c
View file

@ -308,10 +308,11 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
switch (regs.verdict.code & NF_VERDICT_MASK) {
case NF_ACCEPT:
case NF_DROP:
case NF_QUEUE:
case NF_STOLEN:
return regs.verdict.code;
case NF_DROP:
return NF_DROP_REASON(pkt->skb, SKB_DROP_REASON_NETFILTER_DROP, EPERM);
}
switch (regs.verdict.code) {
@ -342,6 +343,9 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
if (static_branch_unlikely(&nft_counters_enabled))
nft_update_chain_stats(basechain, pkt);
if (nft_base_chain(basechain)->policy == NF_DROP)
return NF_DROP_REASON(pkt->skb, SKB_DROP_REASON_NETFILTER_DROP, EPERM);
return nft_base_chain(basechain)->policy;
}
EXPORT_SYMBOL_GPL(nft_do_chain);