[PATCH net-next] netfilter: xt_owner: no longer acquire sk_callback_lock in mt_owner()

[Eric Dumazet <edumazet@google.com>]

After commit 983512f3a87f ("net: Drop the lock in skb_may_tx_timestamp()")
from Sebastian Andrzej Siewior, apply the same logic in mt_owner()
to avoid touching sk_callback_lock.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/netfilter/xt_owner.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/net/netfilter/xt_owner.c b/net/netfilter/xt_owner.c
index 50332888c8d233aab0915a31f2f616f3171da45e..5845eabe6161b3a90df422e6e1055165d6538791 100644
--- a/net/netfilter/xt_owner.c
+++ b/net/netfilter/xt_owner.c
@@ -63,11 +63,12 @@ static bool
 owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 {
 	const struct xt_owner_match_info *info = par->matchinfo;
-	const struct file *filp;
 	struct sock *sk = skb_to_full_sk(skb);
 	struct net *net = xt_net(par);
+	const struct socket *sock;
+	const struct file *filp;
 
-	if (!sk || !sk->sk_socket || !net_eq(net, sock_net(sk)))
+	if (!sk || !READ_ONCE(sk->sk_socket) || !net_eq(net, sock_net(sk)))
 		return (info->match ^ info->invert) == 0;
 	else if (info->match & info->invert & XT_OWNER_SOCKET)
 		/*
@@ -76,10 +77,16 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		 */
 		return false;
 
-	read_lock_bh(&sk->sk_callback_lock);
-	filp = sk->sk_socket ? sk->sk_socket->file : NULL;
+	/* The sk pointer remains valid as long as the skb is. The sk_socket and
+	 * file pointer may become NULL if the socket is closed. Both structures
+	 * (including file->cred) are RCU freed which means they can be accessed
+	 * within a RCU read section.
+	 */
+	rcu_read_lock();
+	sock = READ_ONCE(sk->sk_socket);
+	filp = sock ? READ_ONCE(sock->file) : NULL;
 	if (filp == NULL) {
-		read_unlock_bh(&sk->sk_callback_lock);
+		rcu_read_unlock();
 		return ((info->match ^ info->invert) &
 		       (XT_OWNER_UID | XT_OWNER_GID)) == 0;
 	}
@@ -90,7 +97,7 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		if ((uid_gte(filp->f_cred->fsuid, uid_min) &&
 		     uid_lte(filp->f_cred->fsuid, uid_max)) ^
 		    !(info->invert & XT_OWNER_UID)) {
-			read_unlock_bh(&sk->sk_callback_lock);
+			rcu_read_unlock();
 			return false;
 		}
 	}
@@ -118,12 +125,12 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		}
 
 		if (match ^ !(info->invert & XT_OWNER_GID)) {
-			read_unlock_bh(&sk->sk_callback_lock);
+			rcu_read_unlock();
 			return false;
 		}
 	}
 
-	read_unlock_bh(&sk->sk_callback_lock);
+	rcu_read_unlock();
 	return true;
 }
 
-- 
2.53.0.371.g1d285c8824-goog