[PATCH nf-next] netfilter: nft_meta: no longer acquire sk_callback_lock in nft_meta_get_eval_skugid()

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/netfilter/nft_meta.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/net/netfilter/nft_meta.c b/net/netfilter/nft_meta.c
index 983158274c68dc6626032e6f55afc452c4be443f..d0df6cf374d1abe83ce2a02d93fb366e12b3d8d2 100644
--- a/net/netfilter/nft_meta.c
+++ b/net/netfilter/nft_meta.c
@@ -131,33 +131,36 @@ nft_meta_get_eval_skugid(enum nft_meta_keys key,
 			 u32 *dest,
 			 const struct nft_pktinfo *pkt)
 {
-	struct sock *sk = skb_to_full_sk(pkt->skb);
-	struct socket *sock;
+	const struct sock *sk = skb_to_full_sk(pkt->skb);
+	const struct socket *sock;
+	const struct file *file;
 
 	if (!sk || !sk_fullsock(sk) || !net_eq(nft_net(pkt), sock_net(sk)))
 		return false;
 
-	read_lock_bh(&sk->sk_callback_lock);
-	sock = sk->sk_socket;
-	if (!sock || !sock->file) {
-		read_unlock_bh(&sk->sk_callback_lock);
+	/* 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)
 		return false;
-	}
 
 	switch (key) {
 	case NFT_META_SKUID:
 		*dest = from_kuid_munged(sock_net(sk)->user_ns,
-					 sock->file->f_cred->fsuid);
+					 file->f_cred->fsuid);
 		break;
 	case NFT_META_SKGID:
 		*dest =	from_kgid_munged(sock_net(sk)->user_ns,
-					 sock->file->f_cred->fsgid);
+					 file->f_cred->fsgid);
 		break;
 	default:
 		break;
 	}
 
-	read_unlock_bh(&sk->sk_callback_lock);
 	return true;
 }
 
-- 
2.53.0.414.gf7e9f6c205-goog