[PATCH nf-next] netfilter: nfnetlink_log: no longer acquire sk_callback_lock

[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
__build_packet_message() to avoid touching sk->sk_callback_lock.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/netfilter/nfnetlink_log.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/nfnetlink_log.c b/net/netfilter/nfnetlink_log.c
index bfcb9cd335bff59fa4d0d09eb52943566e4098e7..bef28e5230bb3b4d4b1ef4d5111909bac969b343 100644
--- a/net/netfilter/nfnetlink_log.c
+++ b/net/netfilter/nfnetlink_log.c
@@ -611,19 +611,26 @@ __build_packet_message(struct nfnl_log_net *log,
 	/* UID */
 	sk = skb->sk;
 	if (sk && sk_fullsock(sk)) {
-		read_lock_bh(&sk->sk_callback_lock);
-		if (sk->sk_socket && sk->sk_socket->file) {
-			struct file *file = sk->sk_socket->file;
+		const struct socket *sock;
+		const struct file *file;
+
+		/* 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.
+		 */
+		sock = READ_ONCE(sk->sk_socket);
+		file = sock ? READ_ONCE(sock->file) : NULL;
+		if (file) {
 			const struct cred *cred = file->f_cred;
 			struct user_namespace *user_ns = inst->peer_user_ns;
 			__be32 uid = htonl(from_kuid_munged(user_ns, cred->fsuid));
 			__be32 gid = htonl(from_kgid_munged(user_ns, cred->fsgid));
-			read_unlock_bh(&sk->sk_callback_lock);
 			if (nla_put_be32(inst->skb, NFULA_UID, uid) ||
 			    nla_put_be32(inst->skb, NFULA_GID, gid))
 				goto nla_put_failure;
-		} else
-			read_unlock_bh(&sk->sk_callback_lock);
+		}
 	}
 
 	/* local sequence number */
-- 
2.53.0.414.gf7e9f6c205-goog