[PATCH nf] netfilter: nf_queue: be more careful with sk refcounts

[Florian Westphal <fw@strlen.de>]

Eric Dumazet says:
  The sock_hold() side seems suspect, because there is no guarantee
  that sk_refcnt is not already 0.

Also, there is a way to wire up skb->sk in a way that skb doesn't hold
a reference on the socket, so we need to check for that as well.

For refcount-less skb->sk case, try to increment the reference count
and then override the destructor.

On failure, we cannot queue the packet and need to indicate an
error.  THe packet will be dropped by the caller.

Cc: Joe Stringer <joe@cilium.io>
Fixes: 271b72c7fa82c ("udp: RCU handling for Unicast packets.")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Eric, does that look sane to you?
 Joe, it would be nice if you could check the 'skb_sk_is_prefetched' part.

 Thanks!

 include/net/netfilter/nf_queue.h |  2 +-
 net/netfilter/nf_queue.c         | 25 +++++++++++++++++++++----
 net/netfilter/nfnetlink_queue.c  | 12 +++++++++---
 3 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/include/net/netfilter/nf_queue.h b/include/net/netfilter/nf_queue.h
index 9eed51e920e8..980daa6e1e3a 100644
--- a/include/net/netfilter/nf_queue.h
+++ b/include/net/netfilter/nf_queue.h
@@ -37,7 +37,7 @@ void nf_register_queue_handler(const struct nf_queue_handler *qh);
 void nf_unregister_queue_handler(void);
 void nf_reinject(struct nf_queue_entry *entry, unsigned int verdict);
 
-void nf_queue_entry_get_refs(struct nf_queue_entry *entry);
+bool nf_queue_entry_get_refs(struct nf_queue_entry *entry);
 void nf_queue_entry_free(struct nf_queue_entry *entry);
 
 static inline void init_hashrandom(u32 *jhash_initval)
diff --git a/net/netfilter/nf_queue.c b/net/netfilter/nf_queue.c
index 5ab0680db445..63d1516816b1 100644
--- a/net/netfilter/nf_queue.c
+++ b/net/netfilter/nf_queue.c
@@ -96,19 +96,21 @@ static void __nf_queue_entry_init_physdevs(struct nf_queue_entry *entry)
 }
 
 /* Bump dev refs so they don't vanish while packet is out */
-void nf_queue_entry_get_refs(struct nf_queue_entry *entry)
+bool nf_queue_entry_get_refs(struct nf_queue_entry *entry)
 {
 	struct nf_hook_state *state = &entry->state;
 
+	if (state->sk && !refcount_inc_not_zero(&state->sk->sk_refcnt))
+		return false;
+
 	dev_hold(state->in);
 	dev_hold(state->out);
-	if (state->sk)
-		sock_hold(state->sk);
 
 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 	dev_hold(entry->physin);
 	dev_hold(entry->physout);
 #endif
+	return true;
 }
 EXPORT_SYMBOL_GPL(nf_queue_entry_get_refs);
 
@@ -178,6 +180,18 @@ static int __nf_queue(struct sk_buff *skb, const struct nf_hook_state *state,
 		break;
 	}
 
+	if (skb_sk_is_prefetched(skb)) {
+		struct sock *sk = skb->sk;
+
+		if (!sk_is_refcounted(sk)) {
+			if (!refcount_inc_not_zero(&sk->sk_refcnt))
+				return -ENOTCONN;
+
+			/* drop refcount on skb_orphan */
+			skb->destructor = sock_edemux;
+		}
+	}
+
 	entry = kmalloc(sizeof(*entry) + route_key_size, GFP_ATOMIC);
 	if (!entry)
 		return -ENOMEM;
@@ -196,7 +210,10 @@ static int __nf_queue(struct sk_buff *skb, const struct nf_hook_state *state,
 
 	__nf_queue_entry_init_physdevs(entry);
 
-	nf_queue_entry_get_refs(entry);
+	if (!nf_queue_entry_get_refs(entry)) {
+		kfree(entry);
+		return -ENOTCONN;
+	}
 
 	switch (entry->state.pf) {
 	case AF_INET:
diff --git a/net/netfilter/nfnetlink_queue.c b/net/netfilter/nfnetlink_queue.c
index ea2d9c2a44cf..64a6acb6aeae 100644
--- a/net/netfilter/nfnetlink_queue.c
+++ b/net/netfilter/nfnetlink_queue.c
@@ -710,9 +710,15 @@ static struct nf_queue_entry *
 nf_queue_entry_dup(struct nf_queue_entry *e)
 {
 	struct nf_queue_entry *entry = kmemdup(e, e->size, GFP_ATOMIC);
-	if (entry)
-		nf_queue_entry_get_refs(entry);
-	return entry;
+
+	if (!entry)
+		return NULL;
+
+	if (nf_queue_entry_get_refs(entry))
+		return entry;
+
+	kfree(entry);
+	return NULL;
 }
 
 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
-- 
2.34.1