VMCI: fix NULL pointer dereference when unmapping queue pair

I got a NULL pointer dereference report when doing fuzz test:

Call Trace:
  qp_release_pages+0xae/0x130
  qp_host_unregister_user_memory.isra.25+0x2d/0x80
  vmci_qp_broker_unmap+0x191/0x320
  ? vmci_host_do_alloc_queuepair.isra.9+0x1c0/0x1c0
  vmci_host_unlocked_ioctl+0x59f/0xd50
  ? do_vfs_ioctl+0x14b/0xa10
  ? tomoyo_file_ioctl+0x28/0x30
  ? vmci_host_do_alloc_queuepair.isra.9+0x1c0/0x1c0
  __x64_sys_ioctl+0xea/0x120
  do_syscall_64+0x34/0xb0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

When a queue pair is created by the following call, it will not
register the user memory if the page_store is NULL, and the
entry->state will be set to VMCIQPB_CREATED_NO_MEM.

vmci_host_unlocked_ioctl
  vmci_host_do_alloc_queuepair
    vmci_qp_broker_alloc
      qp_broker_alloc
        qp_broker_create // set entry->state = VMCIQPB_CREATED_NO_MEM;

When unmapping this queue pair, qp_host_unregister_user_memory() will
be called to unregister the non-existent user memory, which will
result in a null pointer reference. It will also change
VMCIQPB_CREATED_NO_MEM to VMCIQPB_CREATED_MEM, which should not be
present in this operation.

Only when the qp broker has mem, it can unregister the user
memory when unmapping the qp broker.

Only when the qp broker has no mem, it can register the user
memory when mapping the qp broker.

Fixes: 06164d2b ("VMCI: queue pairs implementation.")
Cc: stable <stable@vger.kernel.org>
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Link: https://lore.kernel.org/r/20210818124845.488312-1-wanghai38@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/vmw_vmci/vmci_queue_pair.c

@@ -2243,7 +2243,8 @@ int vmci_qp_broker_map(struct vmci_handle handle,
 
 	result = VMCI_SUCCESS;
 
-	if (context_id != VMCI_HOST_CONTEXT_ID) {
+	if (context_id != VMCI_HOST_CONTEXT_ID &&
+	    !QPBROKERSTATE_HAS_MEM(entry)) {
 		struct vmci_qp_page_store page_store;
 
 		page_store.pages = guest_mem;
@@ -2350,7 +2351,8 @@ int vmci_qp_broker_unmap(struct vmci_handle handle,
 		goto out;
 	}
 
-	if (context_id != VMCI_HOST_CONTEXT_ID) {
+	if (context_id != VMCI_HOST_CONTEXT_ID &&
+	    QPBROKERSTATE_HAS_MEM(entry)) {
 		qp_acquire_queue_mutex(entry->produce_q);
 		result = qp_save_headers(entry);
 		if (result < VMCI_SUCCESS)