Merge branch 'tuntap-socket-uid'

Pietro Borrello says:

====================
tuntap: correctly initialize socket uid

sock_init_data() assumes that the `struct socket` passed in input is
contained in a `struct socket_alloc` allocated with sock_alloc().
However, tap_open() and tun_chr_open() pass a `struct socket` embedded
in a `struct tap_queue` and `struct tun_file` respectively, both
allocated with sk_alloc().
This causes a type confusion when issuing a container_of() with
SOCK_INODE() in sock_init_data() which results in assigning a wrong
sk_uid to the `struct sock` in input.

Due to the type confusion, both sockets happen to have their uid set
to 0, i.e. root.
While it will be often correct, as tuntap devices require
CAP_NET_ADMIN, it may not always be the case.
Not sure how widespread is the impact of this, it seems the socket uid
may be used for network filtering and routing, thus tuntap sockets may
be incorrectly managed.
Additionally, it seems a socket with an incorrect uid may be returned
to the vhost driver when issuing a get_socket() on a tuntap device in
vhost_net_set_backend().

Fix the bugs by adding and using sock_init_data_uid(), which
explicitly takes a uid as argument.
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
---
Changes in v3:
- Fix the bug by defining and using sock_init_data_uid()
- Link to v2: https://lore.kernel.org/r/20230131-tuntap-sk-uid-v2-0-29ec15592813@diag.uniroma1.it

Changes in v2:
- Shorten and format comments
- Link to v1: https://lore.kernel.org/r/20230131-tuntap-sk-uid-v1-0-af4f9f40979d@diag.uniroma1.it
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tap.c

@@ -533,7 +533,7 @@ static int tap_open(struct inode *inode, struct file *file)
 	q->sock.state = SS_CONNECTED;
 	q->sock.file = file;
 	q->sock.ops = &tap_socket_ops;
-	sock_init_data(&q->sock, &q->sk);
+	sock_init_data_uid(&q->sock, &q->sk, inode->i_uid);
 	q->sk.sk_write_space = tap_sock_write_space;
 	q->sk.sk_destruct = tap_sock_destruct;
 	q->flags = IFF_VNET_HDR | IFF_NO_PI | IFF_TAP;

drivers/net/tun.c

@@ -3448,7 +3448,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 	tfile->socket.file = file;
 	tfile->socket.ops = &tun_socket_ops;
 
-	sock_init_data(&tfile->socket, &tfile->sk);
+	sock_init_data_uid(&tfile->socket, &tfile->sk, inode->i_uid);
 
 	tfile->sk.sk_write_space = tun_sock_write_space;
 	tfile->sk.sk_sndbuf = INT_MAX;

include/net/sock.h

@@ -1956,7 +1956,12 @@ void sk_common_release(struct sock *sk);
  *	Default socket callbacks and setup code
  */
 
-/* Initialise core socket variables */
+/* Initialise core socket variables using an explicit uid. */
+void sock_init_data_uid(struct socket *sock, struct sock *sk, kuid_t uid);
+
+/* Initialise core socket variables.
+ * Assumes struct socket *sock is embedded in a struct socket_alloc.
+ */
 void sock_init_data(struct socket *sock, struct sock *sk);
 
 /*

net/core/sock.c

@@ -3383,7 +3383,7 @@ void sk_stop_timer_sync(struct sock *sk, struct timer_list *timer)
 }
 EXPORT_SYMBOL(sk_stop_timer_sync);
 
-void sock_init_data(struct socket *sock, struct sock *sk)
+void sock_init_data_uid(struct socket *sock, struct sock *sk, kuid_t uid)
 {
 	sk_init_common(sk);
 	sk->sk_send_head	=	NULL;
@@ -3403,11 +3403,10 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 		sk->sk_type	=	sock->type;
 		RCU_INIT_POINTER(sk->sk_wq, &sock->wq);
 		sock->sk	=	sk;
-		sk->sk_uid	=	SOCK_INODE(sock)->i_uid;
 	} else {
 		RCU_INIT_POINTER(sk->sk_wq, NULL);
-		sk->sk_uid	=	make_kuid(sock_net(sk)->user_ns, 0);
 	}
+	sk->sk_uid	=	uid;
 
 	rwlock_init(&sk->sk_callback_lock);
 	if (sk->sk_kern_sock)
@@ -3466,6 +3465,16 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 	refcount_set(&sk->sk_refcnt, 1);
 	atomic_set(&sk->sk_drops, 0);
 }
+EXPORT_SYMBOL(sock_init_data_uid);
+
+void sock_init_data(struct socket *sock, struct sock *sk)
+{
+	kuid_t uid = sock ?
+		SOCK_INODE(sock)->i_uid :
+		make_kuid(sock_net(sk)->user_ns, 0);
+
+	sock_init_data_uid(sock, sk, uid);
+}
 EXPORT_SYMBOL(sock_init_data);
 
 void lock_sock_nested(struct sock *sk, int subclass)