diff --git a/Source/Core/Core/IOS/USB/USBV5.cpp b/Source/Core/Core/IOS/USB/USBV5.cpp
@@ -8,6 +8,7 @@
 #include <cstddef>
 #include <cstring>
 
+#include "Common/Assert.h"
 #include "Common/ChunkFile.h"
 #include "Common/Logging/Log.h"
 #include "Common/Swap.h"
@@ -31,14 +32,14 @@ V5CtrlMessage::V5CtrlMessage(Kernel& ios, const IOCtlVRequest& ioctlv)
 V5BulkMessage::V5BulkMessage(Kernel& ios, const IOCtlVRequest& ioctlv)
     : BulkMessage(ios, ioctlv, ioctlv.GetVector(1)->address)
 {
-  length = static_cast<u16>(ioctlv.GetVector(1)->size);
+  length = ioctlv.GetVector(1)->size;
   endpoint = Memory::Read_U8(ioctlv.in_vectors[0].address + 18);
 }
 
 V5IntrMessage::V5IntrMessage(Kernel& ios, const IOCtlVRequest& ioctlv)
     : IntrMessage(ios, ioctlv, ioctlv.GetVector(1)->address)
 {
-  length = static_cast<u16>(ioctlv.GetVector(1)->size);
+  length = ioctlv.GetVector(1)->size;
   endpoint = Memory::Read_U8(ioctlv.in_vectors[0].address + 14);
 }
 
@@ -48,9 +49,16 @@ V5IsoMessage::V5IsoMessage(Kernel& ios, const IOCtlVRequest& ioctlv)
   num_packets = Memory::Read_U8(ioctlv.in_vectors[0].address + 16);
   endpoint = Memory::Read_U8(ioctlv.in_vectors[0].address + 17);
   packet_sizes_addr = ioctlv.GetVector(1)->address;
+  u32 total_packet_size = 0;
   for (size_t i = 0; i < num_packets; ++i)
-    packet_sizes.push_back(Memory::Read_U16(static_cast<u32>(packet_sizes_addr + i * sizeof(u16))));
-  length = static_cast<u16>(ioctlv.GetVector(2)->size);
+  {
+    const u32 packet_size = Memory::Read_U16(static_cast<u32>(packet_sizes_addr + i * sizeof(u16)));
+    packet_sizes.push_back(packet_size);
+    total_packet_size += packet_size;
+  }
+  length = ioctlv.GetVector(2)->size;
+  ASSERT_MSG(IOS_USB, length == total_packet_size, "Wrong buffer size (0x%x != 0x%x)", length,
+             total_packet_size);
 }
 }  // namespace USB
 
@@ -132,7 +140,7 @@ IPCCommandResult USBV5ResourceManager::SetAlternateSetting(USBV5Device& device,
                                                            const IOCtlRequest& request)
 {
   const auto host_device = GetDeviceById(device.host_id);
-  if (!host_device->Attach(device.interface_number))
+  if (!host_device->AttachAndChangeInterface(device.interface_number))
     return GetDefaultReply(-1);
 
   const u8 alt_setting = Memory::Read_U8(request.buffer_in + 2 * sizeof(s32));

diff --git a/Source/Core/Core/IOS/USB/USB_HID/HIDv4.cpp b/Source/Core/Core/IOS/USB/USB_HID/HIDv4.cpp
@@ -55,7 +55,7 @@ IPCCommandResult USB_HIDv4::IOCtl(const IOCtlRequest& request)
     if (request.buffer_in == 0 || request.buffer_in_size != 32)
       return GetDefaultReply(IPC_EINVAL);
     const auto device = GetDeviceByIOSID(Memory::Read_U32(request.buffer_in + 16));
-    if (!device->Attach(0))
+    if (!device->Attach())
       return GetDefaultReply(IPC_EINVAL);
     return HandleTransfer(device, request.request,
                           [&, this]() { return SubmitTransfer(*device, request); });

diff --git a/Source/Core/Core/IOS/USB/USB_HID/HIDv5.cpp b/Source/Core/Core/IOS/USB/USB_HID/HIDv5.cpp
@@ -67,7 +67,10 @@ IPCCommandResult USB_HIDv5::IOCtlV(const IOCtlVRequest& request)
     if (!device)
       return GetDefaultReply(IPC_EINVAL);
     auto host_device = GetDeviceById(device->host_id);
-    host_device->Attach(device->interface_number);
+    if (request.request == USB::IOCTLV_USBV5_CTRLMSG)
+      host_device->Attach();
+    else
+      host_device->AttachAndChangeInterface(device->interface_number);
     return HandleTransfer(host_device, request.request,
                           [&, this]() { return SubmitTransfer(*device, *host_device, request); });
   }

diff --git a/Source/Core/Core/IOS/USB/USB_VEN/VEN.cpp b/Source/Core/Core/IOS/USB/USB_VEN/VEN.cpp
@@ -76,7 +76,10 @@ IPCCommandResult USB_VEN::IOCtlV(const IOCtlVRequest& request)
     if (!device)
       return GetDefaultReply(IPC_EINVAL);
     auto host_device = GetDeviceById(device->host_id);
-    host_device->Attach(device->interface_number);
+    if (request.request == USB::IOCTLV_USBV5_CTRLMSG)
+      host_device->Attach();
+    else
+      host_device->AttachAndChangeInterface(device->interface_number);
     return HandleTransfer(host_device, request.request,
                           [&, this]() { return SubmitTransfer(*host_device, request); });
   }
@@ -104,8 +107,10 @@ s32 USB_VEN::SubmitTransfer(USB::Device& device, const IOCtlVRequest& ioctlv)
 
 IPCCommandResult USB_VEN::CancelEndpoint(USBV5Device& device, const IOCtlRequest& request)
 {
-  const u8 endpoint = static_cast<u8>(Memory::Read_U32(request.buffer_in + 8));
-  GetDeviceById(device.host_id)->CancelTransfer(endpoint);
+  const u8 endpoint = Memory::Read_U8(request.buffer_in + 8);
+  // IPC_EINVAL (-4) is returned when no transfer was cancelled.
+  if (GetDeviceById(device.host_id)->CancelTransfer(endpoint) < 0)
+    return GetDefaultReply(IPC_EINVAL);
   return GetDefaultReply(IPC_SUCCESS);
 }