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vgpu_unlock

Unlock vGPU functionality for consumer grade GPUs.

Important!

This tool is very untested, use at your own risk.

Description

This tool enables the use of Geforce and Quadro GPUs with the NVIDIA vGPU software. NVIDIA vGPU normally only supports a few Tesla GPUs but since some Geforce and Quadro GPUs share the same physical chip as the Tesla this is only a software limitation for those GPUs. This tool aims to remove this limitation.

Dependencies:

  • This tool requires Python3, the latest version is recommended.
  • The python package "frida" is required. pip3 install frida.
  • The tool requires the NVIDIA GRID vGPU driver.
  • "dkms" is highly recommended as it simplifies the process of rebuilding the driver alot.

Installation:

In the following instructions <path_to_vgpu_unlock> need to be replaced with the path to this repository on the target system and <version> need to be replaced with the version of the NVIDIA GRID vGPU driver.

Install the NVIDIA GRID vGPU driver, make sure to install it as a dkms module.

./nvidia-installer

Modify the line begining with ExecStart= in /lib/systemd/system/nvidia-vgpud.service and /lib/systemd/system/nvidia-vgpu-mgr.service to use vgpu_unlock as executable and pass the original executable as the first argument. Ex:

ExecStart=<path_to_vgpu_unlock>/vgpu_unlock /usr/bin/nvidia-vgpud

Reload the systemd daemons:

systemctl daemon-reload

Modify the file /usr/src/nvidia-<version>/nvidia/os-interface.c and add the following line after the lines begining with #include at the start of the file.

#include "<path_to_vgpu_unlock>/vgpu_unlock_hooks.c"

Modify the file /usr/src/nvidia-<version>/nvidia/nvidia.Kbuild and add the following line.

ldflags-y += -T <path_to_vgpu_unlock>/kern.ld

Remove the nvidia kernel module using dkms:

dkms remove -m nvidia -v <version> --all

Rebuild and reinstall the nvidia kernel module using dkms:

dkms install -m nvidia -v <version>

Reboot.

NOTE

This script will only work if there exists a vGPU compatible Tesla GPU that uses the same physical chip as the actual GPU being used.

How it works

vGPU supported?

In order to determine if a certain GPU supports the vGPU functionality the driver looks at the PCI device ID. This identifier together with the PCI vendor ID is unique for each type of PCI device. In order to enable vGPU support we need to tell the driver that the PCI device ID of the installed GPU is one of the device IDs used by a vGPU capable GPU.

Userspace script: vgpu_unlock

The userspace services nvidia-vgpud and nvidia-vgpu-mgr uses the ioctl syscall to communicate with the kernel module. Specifically they read the PCI device ID and determines if the installed GPU is vGPU capable.

The python script vgpu_unlock intercepts all ioctl syscalls between the executable specified as the first argument and the kernel. The script then modifies the kernel responses to indicate a PCI device ID with vGPU support and a vGPU capable GPU.

Kernel module hooks: vgpu_unlock_hooks.c

In order to exchange data with the GPU the kernel module maps the physical address space of the PCI bus into its own virtual address space. This is done using the ioremap* kernel functions. The kernel module then reads and writes data into that mapped address space. This is done using the memcpy kernel function.

By including the vgpu_unlock_hooks.c file into the os-interface.c file we can use C preprocessor macros to replace and intercept calls to the iormeap and memcpy functions. Doing this allows us to maintain a view of what is mapped where and what data that is being accessed.

Kernel module linker script: kern.ld

This is a modified version of the default linker script provided by gcc. The script is modified to place the .rodata section of nv-kernel.o into .data section instead of .rodata, making it writable. The script also provide the symbols vgpu_unlock_nv_kern_rodata_beg and vgpu_unlock_nv_kern_rodata_end to let us know where that section begins and ends.

How it all comes together

After boot the nvidia-vgpud service queries the kernel for all installed GPUs and checks for vGPU capability. This call is intercepted by the vgpu_unlock python script and the GPU is made vGPU capable. If a vGPU capable GPU is found then nvidia-vgpu creates an MDEV device and the /sys/class/mdev_bus directory is created by the system.

vGPU devices can now be created by echoing UUIDs into the create files in the mdev bus representation. This will create additional structures representing the new vGPU device on the MDEV bus. These devices can then be assigned to VMs, and when the VM starts it will open the MDEV device. This causes nvidia-vgpu-mgr to start communicating with the kernel using ioctl. Again these calls are intercepted by the vgpu_unlock python script and when nvidia-vgpu-mgr asks if the GPU is vGPU capable the answer is changed to yes. After that check it attempts to initialize the vGPU device instance.

Initialization of the vGPU device is handled by the kernel module and it performs its own check for vGPU capability, this one is a bit more complicated.

The kernel module maps the physical PCI address range 0xf0000000-0xf1000000 into its virtual address space, it then performs some magical operations which we don't really know what they do. What we do know is that after these operations it accesses a 128 bit value at physical address 0xf0029624, which we call the magic value. The kernel module also accessses a 128 bit value at physical address 0xf0029634, which we call the key value.

The kernel module then has a couple of lookup tables for the magic value, one for vGPU capable GPUs and one for the others. So the kernel module looks for the magic value in both of these lookup tables, and if it is found that table entry also contains a set of AES-128 encrypted data blocks and a HMAC-SHA256 signature.

The signature is then validated by using the key value mentioned earlier to calculate the HMAC-SHA256 signature over the encrypted data blocks. If the signature is correct, then the blocks are decrypted using AES-128 and the same key.

Inside of the decrypted data is once again the PCI device ID.

So in order for the kernel module to accept the GPU as vGPU capable the magic value will have to be in the table of vGPU capable magic values, the key has to generate a valid HMAC-SHA256 signature and the AES-128 decrypted data blocks has to contain a vGPU capable PCI device ID. If any of these checks fail, then the error code 0x56 "Call not supported" is returned.

In order to make these checks pass the hooks in vgpu_unlock_hooks.c will look for a ioremap call that maps the physical address range that contain the magic and key values, recalculate the addresses of those values into the virtual address space of the kernel module, monitor memcpy operations reading at those addresses, and if such an operation occurs, keep a copy of the value until both are known, locate the lookup tables in the .rodata section of nv-kernel.o, find the signature and data bocks, validate the signature, decrypt the blocks, edit the PCI device ID in the decrypted data, reencrypt the blocks, regenerate the signature and insert the magic, blocks and signature into the table of vGPU capable magic values. And that's what they do.