[PATCH nf-next] netfilter: nfnetlink_queue: 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
nfqnl_put_sk_uidgid() to avoid touching sk->sk_callback_lock.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/netfilter/nfnetlink_queue.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/nfnetlink_queue.c b/net/netfilter/nfnetlink_queue.c
index f1c8049861a6b7c91c2d1fd0aa1ddb2db9a31d8f..fbe5a8d7143358906d8e77d3e7a62152fdac337b 100644
--- a/net/netfilter/nfnetlink_queue.c
+++ b/net/netfilter/nfnetlink_queue.c
@@ -545,14 +545,23 @@ nfqnl_put_packet_info(struct sk_buff *nlskb, struct sk_buff *packet,
 
 static int nfqnl_put_sk_uidgid(struct sk_buff *skb, struct sock *sk)
 {
+	const struct socket *sock;
+	const struct file *file;
 	const struct cred *cred;
 
 	if (!sk_fullsock(sk))
 		return 0;
 
-	read_lock_bh(&sk->sk_callback_lock);
-	if (sk->sk_socket && sk->sk_socket->file) {
-		cred = sk->sk_socket->file->f_cred;
+	/* 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) {
+		cred = file->f_cred;
 		if (nla_put_be32(skb, NFQA_UID,
 		    htonl(from_kuid_munged(&init_user_ns, cred->fsuid))))
 			goto nla_put_failure;
@@ -560,11 +569,9 @@ static int nfqnl_put_sk_uidgid(struct sk_buff *skb, struct sock *sk)
 		    htonl(from_kgid_munged(&init_user_ns, cred->fsgid))))
 			goto nla_put_failure;
 	}
-	read_unlock_bh(&sk->sk_callback_lock);
 	return 0;
 
 nla_put_failure:
-	read_unlock_bh(&sk->sk_callback_lock);
 	return -1;
 }
 
-- 
2.53.0.414.gf7e9f6c205-goog