net: core: Add a UID field to struct sock.

Protocol sockets (struct sock) don't have UIDs, but most of the
time, they map 1:1 to userspace sockets (struct socket) which do.

Various operations such as the iptables xt_owner match need
access to the "UID of a socket", and do so by following the
backpointer to the struct socket. This involves taking
sk_callback_lock and doesn't work when there is no socket
because userspace has already called close().

Simplify this by adding a sk_uid field to struct sock whose value
matches the UID of the corresponding struct socket. The semantics
are as follows:

1. Whenever sk_socket is non-null: sk_uid is the same as the UID
   in sk_socket, i.e., matches the return value of sock_i_uid.
   Specifically, the UID is set when userspace calls socket(),
   fchown(), or accept().
2. When sk_socket is NULL, sk_uid is defined as follows:
   - For a socket that no longer has a sk_socket because
     userspace has called close(): the previous UID.
   - For a cloned socket (e.g., an incoming connection that is
     established but on which userspace has not yet called
     accept): the UID of the socket it was cloned from.
   - For a socket that has never had an sk_socket: UID 0 inside
     the user namespace corresponding to the network namespace
     the socket belongs to.

Kernel sockets created by sock_create_kern are a special case
of #1 and sk_uid is the user that created them. For kernel
sockets created at network namespace creation time, such as the
per-processor ICMP and TCP sockets, this is the user that created
the network namespace.

[Backport of net-next 86741ec25462e4c8cdce6df2f41ead05568c7d5e]

Bug: 16355602
Change-Id: Idbc3e9a0cec91c4c6e01916b967b6237645ebe59
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index ae1509860..1fa662430 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -394,6 +394,7 @@ struct sock {
 	void			*sk_security;
 #endif
 	__u32			sk_mark;
+	kuid_t			sk_uid;
 	u32			sk_classid;
 	struct cg_proto		*sk_cgrp;
 	void			(*sk_state_change)(struct sock *sk);
@@ -1733,6 +1734,7 @@ static inline void sock_graft(struct sock *sk, struct socket *parent)
 	sk->sk_wq = parent->wq;
 	parent->sk = sk;
 	sk_set_socket(sk, parent);
+	sk->sk_uid = SOCK_INODE(parent)->i_uid;
 	security_sock_graft(sk, parent);
 	write_unlock_bh(&sk->sk_callback_lock);
 }
@@ -1740,6 +1742,11 @@ static inline void sock_graft(struct sock *sk, struct socket *parent)
 extern kuid_t sock_i_uid(struct sock *sk);
 extern unsigned long sock_i_ino(struct sock *sk);
 
+static inline kuid_t sock_net_uid(const struct net *net, const struct sock *sk)
+{
+	return sk ? sk->sk_uid : make_kuid(net->user_ns, 0);
+}
+
 static inline struct dst_entry *
 __sk_dst_get(struct sock *sk)
 {