Merge patch series "allow to create pidfds for reaped tasks with SCM_PIDFD"

This is a logical continuation of the earlier work to create pidfds for
reaped tasks through the SO_PEERPIDFD socket option merged in
923ea4d4482b ("Merge patch series "net, pidfs: enable handing out pidfds
for reaped sk->sk_peer_pid"").

* patches from https://lore.kernel.org/20250703222314.309967-1-aleksandr.mikhalitsyn@canonical.com:
  selftests: net: extend SCM_PIDFD test to cover stale pidfds
  af_unix: enable handing out pidfds for reaped tasks in SCM_PIDFD
  af_unix: stash pidfs dentry when needed
  af_unix/scm: fix whitespace errors
  af_unix: introduce and use scm_replace_pid() helper
  af_unix: introduce unix_skb_to_scm helper
  af_unix: rework unix_maybe_add_creds() to allow sleep

Link: https://lore.kernel.org/20250703222314.309967-1-aleksandr.mikhalitsyn@canonical.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 44 insertions, 13 deletions

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 129388c309b0..d52811321fce 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1929,7 +1929,7 @@ static void unix_destruct_scm(struct sk_buff *skb)
 	struct scm_cookie scm;
 
 	memset(&scm, 0, sizeof(scm));
-	scm.pid  = UNIXCB(skb).pid;
+	scm.pid = UNIXCB(skb).pid;
 	if (UNIXCB(skb).fp)
 		unix_detach_fds(&scm, skb);
 
@@ -1955,21 +1955,45 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
 	return err;
 }
 
-/*
+static void unix_skb_to_scm(struct sk_buff *skb, struct scm_cookie *scm)
+{
+	scm_set_cred(scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
+	unix_set_secdata(scm, skb);
+}
+
+/**
+ * unix_maybe_add_creds() - Adds current task uid/gid and struct pid to skb if needed.
+ * @skb: skb to attach creds to.
+ * @sk: Sender sock.
+ * @other: Receiver sock.
+ *
  * Some apps rely on write() giving SCM_CREDENTIALS
  * We include credentials if source or destination socket
  * asserted SOCK_PASSCRED.
+ *
+ * Context: May sleep.
+ * Return: On success zero, on error a negative error code is returned.
  */
-static void unix_maybe_add_creds(struct sk_buff *skb, const struct sock *sk,
-				 const struct sock *other)
+static int unix_maybe_add_creds(struct sk_buff *skb, const struct sock *sk,
+				const struct sock *other)
 {
 	if (UNIXCB(skb).pid)
-		return;
+		return 0;
 
 	if (unix_may_passcred(sk) || unix_may_passcred(other)) {
-		UNIXCB(skb).pid = get_pid(task_tgid(current));
+		struct pid *pid;
+		int err;
+
+		pid = task_tgid(current);
+		err = pidfs_register_pid(pid);
+		if (unlikely(err))
+			return err;
+
+		UNIXCB(skb).pid = get_pid(pid);
 		current_uid_gid(&UNIXCB(skb).uid, &UNIXCB(skb).gid);
 	}
+
+	return 0;
 }
 
 static bool unix_skb_scm_eq(struct sk_buff *skb,
@@ -2104,6 +2128,10 @@ lookup:
 		goto out_sock_put;
 	}
 
+	err = unix_maybe_add_creds(skb, sk, other);
+	if (err)
+		goto out_sock_put;
+
 restart:
 	sk_locked = 0;
 	unix_state_lock(other);
@@ -2212,7 +2240,6 @@ restart_locked:
 	if (sock_flag(other, SOCK_RCVTSTAMP))
 		__net_timestamp(skb);
 
-	unix_maybe_add_creds(skb, sk, other);
 	scm_stat_add(other, skb);
 	skb_queue_tail(&other->sk_receive_queue, skb);
 	unix_state_unlock(other);
@@ -2256,6 +2283,10 @@ static int queue_oob(struct sock *sk, struct msghdr *msg, struct sock *other,
 	if (err < 0)
 		goto out;
 
+	err = unix_maybe_add_creds(skb, sk, other);
+	if (err)
+		goto out;
+
 	skb_put(skb, 1);
 	err = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, 1);
 
@@ -2275,7 +2306,6 @@ static int queue_oob(struct sock *sk, struct msghdr *msg, struct sock *other,
 		goto out_unlock;
 	}
 
-	unix_maybe_add_creds(skb, sk, other);
 	scm_stat_add(other, skb);
 
 	spin_lock(&other->sk_receive_queue.lock);
@@ -2369,6 +2399,10 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 
 		fds_sent = true;
 
+		err = unix_maybe_add_creds(skb, sk, other);
+		if (err)
+			goto out_free;
+
 		if (unlikely(msg->msg_flags & MSG_SPLICE_PAGES)) {
 			skb->ip_summed = CHECKSUM_UNNECESSARY;
 			err = skb_splice_from_iter(skb, &msg->msg_iter, size,
@@ -2399,7 +2433,6 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 			goto out_free;
 		}
 
-		unix_maybe_add_creds(skb, sk, other);
 		scm_stat_add(other, skb);
 		skb_queue_tail(&other->sk_receive_queue, skb);
 		unix_state_unlock(other);
@@ -2547,8 +2580,7 @@ int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
 
 	memset(&scm, 0, sizeof(scm));
 
-	scm_set_cred(&scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
-	unix_set_secdata(&scm, skb);
+	unix_skb_to_scm(skb, &scm);
 
 	if (!(flags & MSG_PEEK)) {
 		if (UNIXCB(skb).fp)
@@ -2933,8 +2965,7 @@ unlock:
 				break;
 		} else if (unix_may_passcred(sk)) {
 			/* Copy credentials */
-			scm_set_cred(&scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
-			unix_set_secdata(&scm, skb);
+			unix_skb_to_scm(skb, &scm);
 			check_creds = true;
 		}