net: introduce qemu_receive_packet()

Some NIC supports loopback mode and this is done by calling
nc->info->receive() directly which in fact suppresses the effort of
reentrancy check that is done in qemu_net_queue_send().

Unfortunately we can't use qemu_net_queue_send() here since for
loopback there's no sender as peer, so this patch introduce a
qemu_receive_packet() which is used for implementing loopback mode
for a NIC with this check.

NIC that supports loopback mode will be converted to this helper.

This is intended to address CVE-2021-3416.

Cc: Prasad J Pandit <ppandit@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>

include/net/net.h

@@ -144,12 +144,17 @@ void *qemu_get_nic_opaque(NetClientState *nc);
 void qemu_del_net_client(NetClientState *nc);
 typedef void (*qemu_nic_foreach)(NICState *nic, void *opaque);
 void qemu_foreach_nic(qemu_nic_foreach func, void *opaque);
+int qemu_can_receive_packet(NetClientState *nc);
 int qemu_can_send_packet(NetClientState *nc);
 ssize_t qemu_sendv_packet(NetClientState *nc, const struct iovec *iov,
                           int iovcnt);
 ssize_t qemu_sendv_packet_async(NetClientState *nc, const struct iovec *iov,
                                 int iovcnt, NetPacketSent *sent_cb);
 ssize_t qemu_send_packet(NetClientState *nc, const uint8_t *buf, int size);
+ssize_t qemu_receive_packet(NetClientState *nc, const uint8_t *buf, int size);
+ssize_t qemu_receive_packet_iov(NetClientState *nc,
+                                const struct iovec *iov,
+                                int iovcnt);
 ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size);
 ssize_t qemu_send_packet_async(NetClientState *nc, const uint8_t *buf,
                                int size, NetPacketSent *sent_cb);

include/net/queue.h

@@ -55,6 +55,14 @@ void qemu_net_queue_append_iov(NetQueue *queue,
 
 void qemu_del_net_queue(NetQueue *queue);
 
+ssize_t qemu_net_queue_receive(NetQueue *queue,
+                               const uint8_t *data,
+                               size_t size);
+
+ssize_t qemu_net_queue_receive_iov(NetQueue *queue,
+                                   const struct iovec *iov,
+                                   int iovcnt);
+
 ssize_t qemu_net_queue_send(NetQueue *queue,
                             NetClientState *sender,
                             unsigned flags,

net/net.c

@@ -529,6 +529,17 @@ int qemu_set_vnet_be(NetClientState *nc, bool is_be)
 #endif
 }
 
+int qemu_can_receive_packet(NetClientState *nc)
+{
+    if (nc->receive_disabled) {
+        return 0;
+    } else if (nc->info->can_receive &&
+               !nc->info->can_receive(nc)) {
+        return 0;
+    }
+    return 1;
+}
+
 int qemu_can_send_packet(NetClientState *sender)
 {
     int vm_running = runstate_is_running();
@@ -541,13 +552,7 @@ int qemu_can_send_packet(NetClientState *sender)
         return 1;
     }
 
-    if (sender->peer->receive_disabled) {
-        return 0;
-    } else if (sender->peer->info->can_receive &&
-               !sender->peer->info->can_receive(sender->peer)) {
-        return 0;
-    }
-    return 1;
+    return qemu_can_receive_packet(sender->peer);
 }
 
 static ssize_t filter_receive_iov(NetClientState *nc,
@@ -680,6 +685,25 @@ ssize_t qemu_send_packet(NetClientState *nc, const uint8_t *buf, int size)
     return qemu_send_packet_async(nc, buf, size, NULL);
 }
 
+ssize_t qemu_receive_packet(NetClientState *nc, const uint8_t *buf, int size)
+{
+    if (!qemu_can_receive_packet(nc)) {
+        return 0;
+    }
+
+    return qemu_net_queue_receive(nc->incoming_queue, buf, size);
+}
+
+ssize_t qemu_receive_packet_iov(NetClientState *nc, const struct iovec *iov,
+                                int iovcnt)
+{
+    if (!qemu_can_receive_packet(nc)) {
+        return 0;
+    }
+
+    return qemu_net_queue_receive_iov(nc->incoming_queue, iov, iovcnt);
+}
+
 ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
 {
     return qemu_send_packet_async_with_flags(nc, QEMU_NET_PACKET_FLAG_RAW,

net/queue.c

@@ -182,6 +182,28 @@ static ssize_t qemu_net_queue_deliver_iov(NetQueue *queue,
     return ret;
 }
 
+ssize_t qemu_net_queue_receive(NetQueue *queue,
+                               const uint8_t *data,
+                               size_t size)
+{
+    if (queue->delivering) {
+        return 0;
+    }
+
+    return qemu_net_queue_deliver(queue, NULL, 0, data, size);
+}
+
+ssize_t qemu_net_queue_receive_iov(NetQueue *queue,
+                                   const struct iovec *iov,
+                                   int iovcnt)
+{
+    if (queue->delivering) {
+        return 0;
+    }
+
+    return qemu_net_queue_deliver_iov(queue, NULL, 0, iov, iovcnt);
+}
+
 ssize_t qemu_net_queue_send(NetQueue *queue,
                             NetClientState *sender,
                             unsigned flags,