From: Eric Blake <eblake@redhat.com>

Date: Fri, 26 May 2017 22:04:21 -0500

Subject: [PATCH] nbd: Fully initialize client in case of failed negotiation

If a non-NBD client connects to qemu-nbd, we would end up with

a SIGSEGV in nbd_client_put() because we were trying to

unregister the client's association to the export, even though

we skipped inserting the client into that list.  Easy trigger

in two terminals:

$ qemu-nbd -p 30001 --format=raw file

$ nmap 127.0.0.1 -p 30001

nmap claims that it thinks it connected to a pago-services1

server (which probably means nmap could be updated to learn the

NBD protocol and give a more accurate diagnosis of the open

port - but that's not our problem), then terminates immediately,

so our call to nbd_negotiate() fails.  The fix is to reorder

nbd_co_client_start() to ensure that all initialization occurs

before we ever try talking to a client in nbd_negotiate(), so

that the teardown sequence on negotiation failure doesn't fault

while dereferencing a half-initialized object.

While debugging this, I also noticed that nbd_update_server_watch()

called by nbd_client_closed() was still adding a channel to accept

the next client, even when the state was no longer RUNNING.  That

is fixed by making nbd_can_accept() pay attention to the current

state.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1451614

Signed-off-by: Eric Blake <eblake@redhat.com>

Message-Id: <20170527030421.28366-1-eblake@redhat.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

(cherry picked from commit df8ad9f128c15aa0a0ebc7b24e9a22c9775b67af)

---

 nbd/server.c | 8 +++-----

 qemu-nbd.c   | 2 +-

 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c

index 924a1fe2db..edfda84d43 100644

--- a/nbd/server.c

+++ b/nbd/server.c

@@ -1376,16 +1376,14 @@ static coroutine_fn void nbd_co_client_start(void *opaque)

     if (exp) {

         nbd_export_get(exp);

+        QTAILQ_INSERT_TAIL(&exp->clients, client, next);

     }

+    qemu_co_mutex_init(&client->send_lock);

+

     if (nbd_negotiate(data)) {

         client_close(client);

         goto out;

     }

-    qemu_co_mutex_init(&client->send_lock);

-

-    if (exp) {

-        QTAILQ_INSERT_TAIL(&exp->clients, client, next);

-    }

     nbd_client_receive_next_request(client);

diff --git a/qemu-nbd.c b/qemu-nbd.c

index e080fb7c75..b44764eb87 100644

--- a/qemu-nbd.c

+++ b/qemu-nbd.c

@@ -324,7 +324,7 @@ out:

 static int nbd_can_accept(void)

 {

-    return nb_fds < shared;

+    return state == RUNNING && nb_fds < shared;

 }

 static void nbd_export_closed(NBDExport *exp)