netfilter: nf_conntrack_expect: use conntrack GC to reap expectations

This patch replaces the timer API by GC worker approach for
expectations, as it already happened in many other subsystems.

Use the existing conntrack GC worker to iterate over the local list of
expectations in the master conntrack to reap expired expectations.
Check IPS_HELPER_BIT to run GC for expectations, set it on for nft_ct
expectation which nevers sets it. Hold the expectation spinlock while
iterating over the master conntrack expectation list to synchronize with
nf_ct_remove_expectations(). This also performs runtime packet path
garbage collection through the expectation insertion and lookup
functions while walking over one of the chains of the global expectation
hashtables. Unconfirmed conntrack entries are skipped since ct->ext can
be reallocated and dying are skipped since those will be gone soon.
Set on IPS_HELPER_BIT if the helper ct extension is added, then the new
GC worker does not need to bump the ct refcount to check if the ct->ext
helper is available.

This removes the extra bump on the refcount for expectation timers, this
allows to remove several nf_ct_expect_put() calls after the unlink,
after this update only refcount remains at 1 while on the expectation
hashes.

This patch implicitly addresses a race with the existing timer API
allowing an expectation to access a stale exp->master pointer which has
been already released when expectation removal loses races with an
expiring timer, ie. timer_del() reporting false.

Add a new NF_CT_EXPECT_DEAD flag to reap this expectation via GC. This
is needed by nf_conntrack_unexpect_related() which is called in error
paths to invalidate newly created expectations that has been added into
the hashes. These expectactions cannot be inmediately released as GC or
nf_ct_remove_expectations() could race to make it. On expectation
insert, the runtime GC reaps stale expectations before checking the
expectation limit set by policy.

Set current timestamp in nf_ct_expect_alloc(), then add the expectation
policy timeout (or custom timeout specified added on top of this) to
specify the expectation lifetime.

Fixes: bffcaad9afdf ("netfilter: ctnetlink: ensure safe access to master conntrack")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

2 files changed, 14 insertions, 3 deletions

diff --git a/include/net/netfilter/nf_conntrack_expect.h b/include/net/netfilter/nf_conntrack_expect.h
index 80f50fd0f7ad..be4a120d549e 100644
--- a/include/net/netfilter/nf_conntrack_expect.h
+++ b/include/net/netfilter/nf_conntrack_expect.h
@@ -54,8 +54,8 @@ struct nf_conntrack_expect {
 	/* The conntrack of the master connection */
 	struct nf_conn *master;
 
-	/* Timer function; deletes the expectation. */
-	struct timer_list timeout;
+	/* jiffies32 when this expectation expires */
+	u32 timeout;
 
 #if IS_ENABLED(CONFIG_NF_NAT)
 	union nf_inet_addr saved_addr;
@@ -69,6 +69,14 @@ struct nf_conntrack_expect {
 	struct rcu_head rcu;
 };
 
+static inline bool nf_ct_exp_is_expired(const struct nf_conntrack_expect *exp)
+{
+	if (READ_ONCE(exp->flags) & NF_CT_EXPECT_DEAD)
+		return true;
+
+	return (__s32)(READ_ONCE(exp->timeout) - nfct_time_stamp) <= 0;
+}
+
 static inline struct net *nf_ct_exp_net(struct nf_conntrack_expect *exp)
 {
 	return read_pnet(&exp->net);
@@ -130,7 +138,6 @@ static inline void nf_ct_unlink_expect(struct nf_conntrack_expect *exp)
 
 void nf_ct_remove_expectations(struct nf_conn *ct);
 void nf_ct_unexpect_related(struct nf_conntrack_expect *exp);
-bool nf_ct_remove_expect(struct nf_conntrack_expect *exp);
 
 void nf_ct_expect_iterate_destroy(bool (*iter)(struct nf_conntrack_expect *e, void *data), void *data);
 void nf_ct_expect_iterate_net(struct net *net,
@@ -153,5 +160,8 @@ static inline int nf_ct_expect_related(struct nf_conntrack_expect *expect,
 	return nf_ct_expect_related_report(expect, 0, 0, flags);
 }
 
+struct nf_conn_help;
+void nf_ct_expectation_gc(struct nf_conn_help *master_help);
+
 #endif /*_NF_CONNTRACK_EXPECT_H*/
 
diff --git a/include/uapi/linux/netfilter/nf_conntrack_common.h b/include/uapi/linux/netfilter/nf_conntrack_common.h
index 56b6b60a814f..ee51045ae1d6 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_common.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_common.h
@@ -160,6 +160,7 @@ enum ip_conntrack_expect_events {
 #define NF_CT_EXPECT_USERSPACE		0x4
 
 #ifdef __KERNEL__
+#define NF_CT_EXPECT_DEAD		0x8
 #define NF_CT_EXPECT_MASK	(NF_CT_EXPECT_PERMANENT | NF_CT_EXPECT_INACTIVE | \
 				 NF_CT_EXPECT_USERSPACE)
 #endif