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.

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

3 files changed, 25 insertions, 1 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 93331a1492db..cf617ee16723 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -419,6 +419,7 @@ struct sock {
         u32                     sk_max_ack_backlog;
         __u32                   sk_priority;
         __u32                   sk_mark;
+        kuid_t                  sk_uid;
         struct pid              *sk_peer_pid;
         const struct cred       *sk_peer_cred;
         long                    sk_rcvtimeo;
@@ -1664,6 +1665,7 @@ 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);
 }
@@ -1671,6 +1673,11 @@ static inline void sock_graft(struct sock *sk, struct socket *parent)
 kuid_t sock_i_uid(struct sock *sk);
 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 u32 net_tx_rndhash(void)
 {
         u32 v = prandom_u32();
diff --git a/net/core/sock.c b/net/core/sock.c
index d8e4532e89e7..40dbc13453f9 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2460,8 +2460,11 @@ void sock_init_data(struct socket *sock, struct sock *sk)
                 sk->sk_type     =       sock->type;
                 sk->sk_wq       =       sock->wq;
                 sock->sk        =       sk;
-        } else
+                sk->sk_uid      =       SOCK_INODE(sock)->i_uid;
+        } else {
                 sk->sk_wq       =       NULL;
+                sk->sk_uid      =       make_kuid(sock_net(sk)->user_ns, 0);
+        }
 
         rwlock_init(&sk->sk_callback_lock);
         lockdep_set_class_and_name(&sk->sk_callback_lock,
diff --git a/net/socket.c b/net/socket.c
index 970a7ea3fc4a..4ce33c35e606 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -518,8 +518,22 @@ static ssize_t sockfs_listxattr(struct dentry *dentry, char *buffer,
         return used;
 }
 
+int sockfs_setattr(struct dentry *dentry, struct iattr *iattr)
+{
+        int err = simple_setattr(dentry, iattr);
+
+        if (!err) {
+                struct socket *sock = SOCKET_I(d_inode(dentry));
+
+                sock->sk->sk_uid = iattr->ia_uid;
+        }
+
+        return err;
+}
+
 static const struct inode_operations sockfs_inode_ops = {
         .listxattr = sockfs_listxattr,
+        .setattr = sockfs_setattr,
 };
 
 /**