[PATCH 1/1] vsock: fix the race conditions in multi-transport support

[Alexander Popov]

There are multiple similar bugs implicitly introduced by the
commit c0cfa2d8a788fcf4 ("vsock: add multi-transports support") and
commit 6a2c0962105ae8ce ("vsock: prevent transport modules unloading").

The bug pattern:
 [1] vsock_sock.transport pointer is copied to a local variable,
 [2] lock_sock() is called,
 [3] the local variable is used.
VSOCK multi-transport support introduced the race condition:
vsock_sock.transport value may change between [1] and [2].

Let's copy vsock_sock.transport pointer to local variables after
the lock_sock() call.

Signed-off-by: Alexander Popov <alex.po...@linux.com>
---
 net/vmw_vsock/af_vsock.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index d10916ab4526..28edac1f9aa6 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -997,9 +997,12 @@ static __poll_t vsock_poll(struct file *file, struct 
socket *sock,
                        mask |= EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND;
 
        } else if (sock->type == SOCK_STREAM) {
-               const struct vsock_transport *transport = vsk->transport;
+               const struct vsock_transport *transport = NULL;
+
                lock_sock(sk);
 
+               transport = vsk->transport;
+
                /* Listening sockets that have connections in their accept
                 * queue can be read.
                 */
@@ -1082,10 +1085,11 @@ static int vsock_dgram_sendmsg(struct socket *sock, 
struct msghdr *msg,
        err = 0;
        sk = sock->sk;
        vsk = vsock_sk(sk);
-       transport = vsk->transport;
 
        lock_sock(sk);
 
+       transport = vsk->transport;
+
        err = vsock_auto_bind(vsk);
        if (err)
                goto out;
@@ -1544,10 +1548,11 @@ static int vsock_stream_setsockopt(struct socket *sock,
        err = 0;
        sk = sock->sk;
        vsk = vsock_sk(sk);
-       transport = vsk->transport;
 
        lock_sock(sk);
 
+       transport = vsk->transport;
+
        switch (optname) {
        case SO_VM_SOCKETS_BUFFER_SIZE:
                COPY_IN(val);
@@ -1680,7 +1685,6 @@ static int vsock_stream_sendmsg(struct socket *sock, 
struct msghdr *msg,
 
        sk = sock->sk;
        vsk = vsock_sk(sk);
-       transport = vsk->transport;
        total_written = 0;
        err = 0;
 
@@ -1689,6 +1693,8 @@ static int vsock_stream_sendmsg(struct socket *sock, 
struct msghdr *msg,
 
        lock_sock(sk);
 
+       transport = vsk->transport;
+
        /* Callers should not provide a destination with stream sockets. */
        if (msg->msg_namelen) {
                err = sk->sk_state == TCP_ESTABLISHED ? -EISCONN : -EOPNOTSUPP;
@@ -1823,11 +1829,12 @@ vsock_stream_recvmsg(struct socket *sock, struct msghdr 
*msg, size_t len,
 
        sk = sock->sk;
        vsk = vsock_sk(sk);
-       transport = vsk->transport;
        err = 0;
 
        lock_sock(sk);
 
+       transport = vsk->transport;
+
        if (!transport || sk->sk_state != TCP_ESTABLISHED) {
                /* Recvmsg is supposed to return 0 if a peer performs an
                 * orderly shutdown. Differentiate between that case and when a
-- 
2.26.2