fwctl subsystem¶

Author:: Jason Gunthorpe

Overview¶

Modern devices contain extensive amounts of FW, and in many cases, are largely software-defined pieces of hardware. The evolution of this approach is largely a reaction to Moore’s Law where a chip tape out is now highly expensive, and the chip design is extremely large. Replacing fixed HW logic with a flexible and tightly coupled FW/HW combination is an effective risk mitigation against chip respin. Problems in the HW design can be counteracted in device FW. This is especially true for devices which present a stable and backwards compatible interface to the operating system driver (such as NVMe).

The FW layer in devices has grown to incredible size and devices frequently integrate clusters of fast processors to run it. For example, mlx5 devices have over 30MB of FW code, and big configurations operate with over 1GB of FW managed runtime state.

The availability of such a flexible layer has created quite a variety in the industry where single pieces of silicon are now configurable software-defined devices and can operate in substantially different ways depending on the need. Further, we often see cases where specific sites wish to operate devices in ways that are highly specialized and require applications that have been tailored to their unique configuration.

Further, devices have become multi-functional and integrated to the point they no longer fit neatly into the kernel’s division of subsystems. Modern multi-functional devices have drivers, such as bnxt/ice/mlx5/pds, that span many subsystems while sharing the underlying hardware using the auxiliary device system.

All together this creates a challenge for the operating system, where devices have an expansive FW environment that needs robust device-specific debugging support, and FW-driven functionality that is not well suited to “generic” interfaces. fwctl seeks to allow access to the full device functionality from user space in the areas of debuggability, management, and first-boot/nth-boot provisioning.

fwctl is aimed at the common device design pattern where the OS and FW communicate via an RPC message layer constructed with a queue or mailbox scheme. In this case the driver will typically have some layer to deliver RPC messages and collect RPC responses from device FW. The in-kernel subsystem drivers that operate the device for its primary purposes will use these RPCs to build their drivers, but devices also usually have a set of ancillary RPCs that don’t really fit into any specific subsystem. For example, a HW RAID controller is primarily operated by the block layer but also comes with a set of RPCs to administer the construction of drives within the HW RAID.

In the past when devices were more single function, individual subsystems would grow different approaches to solving some of these common problems. For instance monitoring device health, manipulating its FLASH, debugging the FW, provisioning, all have various unique interfaces across the kernel.

fwctl’s purpose is to define a common set of limited rules, described below, that allow user space to securely construct and execute RPCs inside device FW. The rules serve as an agreement between the operating system and FW on how to correctly design the RPC interface. As a uAPI the subsystem provides a thin layer of discovery and a generic uAPI to deliver the RPCs and collect the response. It supports a system of user space libraries and tools which will use this interface to control the device using the device native protocols.

Scope of Action¶

fwctl drivers are strictly restricted to being a way to operate the device FW. It is not an avenue to access random kernel internals, or other operating system SW states.

fwctl instances must operate on a well-defined device function, and the device should have a well-defined security model for what scope within the physical device the function is permitted to access. For instance, the most complex PCIe device today may broadly have several function-level scopes:

A privileged function with full access to the on-device global state and configuration

Multiple hypervisor functions with control over itself and child functions used with VMs

Multiple VM functions tightly scoped within the VM

The device may create a logical parent/child relationship between these scopes. For instance a child VM’s FW may be within the scope of the hypervisor FW. It is quite common in the VFIO world that the hypervisor environment has a complex provisioning/profiling/configuration responsibility for the function VFIO assigns to the VM.

Further, within the function, devices often have RPC commands that fall within some general scopes of action (see enum fwctl_rpc_scope):

Access to function & child configuration, FLASH, etc. that becomes live at a function reset. Access to function & child runtime configuration that is transparent or non-disruptive to any driver or VM.

Read-only access to function debug information that may report on FW objects in the function & child, including FW objects owned by other kernel subsystems.

Write access to function & child debug information strictly compatible with the principles of kernel lockdown and kernel integrity protection. Triggers a kernel Taint.

Full debug device access. Triggers a kernel Taint, requires CAP_SYS_RAWIO.

User space will provide a scope label on each RPC and the kernel must enforce the above CAPs and taints based on that scope. A combination of kernel and FW can enforce that RPCs are placed in the correct scope by user space.

Denied behavior¶

There are many things this interface must not allow user space to do (without a Taint or CAP), broadly derived from the principles of kernel lockdown. Some examples:

DMA to/from arbitrary memory, hang the system, compromise FW integrity with untrusted code, or otherwise compromise device or system security and integrity.

Provide an abnormal “back door” to kernel drivers. No manipulation of kernel objects owned by kernel drivers.

Directly configure or otherwise control kernel drivers. A subsystem kernel driver can react to the device configuration at function reset/driver load time, but otherwise must not be coupled to fwctl.

Operate the HW in a way that overlaps with the core purpose of another primary kernel subsystem, such as read/write to LBAs, send/receive of network packets, or operate an accelerator’s data plane.

fwctl is not a replacement for device direct access subsystems like uacce or VFIO.

Operations exposed through fwctl’s non-taining interfaces should be fully sharable with other users of the device. For instance exposing a RPC through fwctl should never prevent a kernel subsystem from also concurrently using that same RPC or hardware unit down the road. In such cases fwctl will be less important than proper kernel subsystems that eventually emerge. Mistakes in this area resulting in clashes will be resolved in favour of a kernel implementation.

fwctl User API¶

General ioctl format

The ioctl interface follows a general format to allow for extensibility. Each ioctl is passed a structure pointer as the argument providing the size of the structure in the first u32. The kernel checks that any structure space beyond what it understands is 0. This allows userspace to use the backward compatible portion while consistently using the newer, larger, structures.

ioctls use a standard meaning for common errnos:

ENOTTY: The IOCTL number itself is not supported at all

E2BIG: The IOCTL number is supported, but the provided structure has non-zero in a part the kernel does not understand.

EOPNOTSUPP: The IOCTL number is supported, and the structure is understood, however a known field has a value the kernel does not understand or support.

EINVAL: Everything about the IOCTL was understood, but a field is not correct.

ENOMEM: Out of memory.

ENODEV: The underlying device has been hot-unplugged and the FD is
orphaned.

As well as additional errnos, within specific ioctls.

struct fwctl_info¶: ioctl(FWCTL_INFO)

Definition:

struct fwctl_info {
    __u32 size;
    __u32 flags;
    __u32 out_device_type;
    __u32 device_data_len;
    __aligned_u64 out_device_data;
};

Members

size: sizeof(struct fwctl_info)
flags: Must be 0
out_device_type: Returns the type of the device from enum fwctl_device_type
device_data_len: On input the length of the out_device_data memory. On output the size of the kernel’s device_data which may be larger or smaller than the input. Maybe 0 on input.
out_device_data: Pointer to a memory of device_data_len bytes. Kernel will fill the entire memory, zeroing as required.

Description

Returns basic information about this fwctl instance, particularly what driver is being used to define the device_data format.

enum fwctl_rpc_scope¶: Scope of access for the RPC

Constants

FWCTL_RPC_CONFIGURATION

Device configuration access scope

Read/write access to device configuration. When configuration is written to the device it remains in a fully supported state.

FWCTL_RPC_DEBUG_READ_ONLY

Read only access to debug information

Readable debug information. Debug information is compatible with kernel lockdown, and does not disclose any sensitive information. For instance exposing any encryption secrets from this information is forbidden.

FWCTL_RPC_DEBUG_WRITE

Writable access to lockdown compatible debug information

Allows write access to data in the device which may leave a fully supported state. This is intended to permit intensive and possibly invasive debugging. This scope will taint the kernel.

FWCTL_RPC_DEBUG_WRITE_FULL

Write access to all debug information

Allows read/write access to everything. Requires CAP_SYS_RAW_IO, so it is not required to follow lockdown principals. If in doubt debugging should be placed in this scope. This scope will taint the kernel.

Description

Refer to fwctl subsystem for a more detailed discussion of these scopes.

struct fwctl_rpc¶: ioctl(FWCTL_RPC)

Definition:

struct fwctl_rpc {
    __u32 size;
    __u32 scope;
    __u32 in_len;
    __u32 out_len;
    __aligned_u64 in;
    __aligned_u64 out;
};

Members

size: sizeof(struct fwctl_rpc)
scope: One of enum fwctl_rpc_scope, required scope for the RPC
in_len: Length of the in memory
out_len: Length of the out memory
in: Request message in device specific format
out: Response message in device specific format

Description

Deliver a Remote Procedure Call to the device FW and return the response. The call’s parameters and return are marshaled into linear buffers of memory. Any errno indicates that delivery of the RPC to the device failed. Return status originating in the device during a successful delivery must be encoded into out.

The format of the buffers matches the out_device_type from FWCTL_INFO.

struct fwctl_info_mlx5¶: ioctl(FWCTL_INFO) out_device_data

Definition:

struct fwctl_info_mlx5 {
    __u32 uid;
    __u32 uctx_caps;
};

Members

uid: The FW UID this FD is bound to. Each command header will force this value.
uctx_caps: The FW capabilities that are enabled for the uid.

Description

Return basic information about the FW interface available.

struct fwctl_info_pds¶

Definition:

struct fwctl_info_pds {
    __u32 uctx_caps;
};

Members

uctx_caps: bitmap of firmware capabilities

Description

Return basic information about the FW interface available.

enum pds_fwctl_capabilities¶

Constants

PDS_FWCTL_QUERY_CAP: firmware can be queried for information
PDS_FWCTL_SEND_CAP: firmware can be sent commands

struct fwctl_rpc_pds¶

Definition:

struct fwctl_rpc_pds {
    struct {
        __u32 op;
        __u32 ep;
        __u32 rsvd;
        __u32 len;
        __aligned_u64 payload;
    } in;
    struct {
        __u32 retval;
        __u32 rsvd[2];
        __u32 len;
        __aligned_u64 payload;
    } out;
};

Members

in: rpc in parameters
in.op: requested operation code
in.ep: firmware endpoint to operate on
in.rsvd: reserved
in.len: length of payload data
in.payload: address of payload buffer
out: rpc out parameters
out.retval: operation result value
out.rsvd: reserved
out.len: length of result data buffer
out.payload: address of payload data buffer

sysfs Class¶

fwctl has a sysfs class (/sys/class/fwctl/fwctlNN/) and character devices (/dev/fwctl/fwctlNN) with a simple numbered scheme. The character device operates the iotcl uAPI described above.

fwctl devices can be related to driver components in other subsystems through sysfs:

$ ls /sys/class/fwctl/fwctl0/device/infiniband/
ibp0s10f0

$ ls /sys/class/infiniband/ibp0s10f0/device/fwctl/
fwctl0/

$ ls /sys/devices/pci0000:00/0000:00:0a.0/fwctl/fwctl0
dev  device  power  subsystem  uevent

User space Community¶

Drawing inspiration from nvme-cli, participating in the kernel side must come with a user space in a common TBD git tree, at a minimum to usefully operate the kernel driver. Providing such an implementation is a pre-condition to merging a kernel driver.

The goal is to build user space community around some of the shared problems we all have, and ideally develop some common user space programs with some starting themes of:

Device in-field debugging

HW provisioning

VFIO child device profiling before VM boot

Confidential Compute topics (attestation, secure provisioning)

that stretch across all subsystems in the kernel. fwupd is a great example of how an excellent user space experience can emerge out of kernel-side diversity.

fwctl Kernel API¶

int fwctl_register(struct fwctl_device *fwctl)¶: Register a new device to the subsystem

Parameters

struct fwctl_device *fwctl: Previously allocated fwctl_device

Description

On return the device is visible through sysfs and /dev, driver ops may be called.

void fwctl_unregister(struct fwctl_device *fwctl)¶: Unregister a device from the subsystem

Parameters

struct fwctl_device *fwctl: Previously allocated and registered fwctl_device

Description

Undoes fwctl_register(). On return no driver ops will be called. The caller must still call fwctl_put() to free the fwctl.

Unregister will return even if userspace still has file descriptors open. This will call ops->close_uctx() on any open FDs and after return no driver op will be called. The FDs remain open but all fops will return -ENODEV.

The design of fwctl allows this sort of disassociation of the driver from the subsystem primarily by keeping memory allocations owned by the core subsytem. The fwctl_device and fwctl_uctx can both be freed without requiring a driver callback. This allows the module to remain unlocked while FDs are open.

struct fwctl_ops¶: Driver provided operations

Definition:

struct fwctl_ops {
    enum fwctl_device_type device_type;
    size_t uctx_size;
    int (*open_uctx)(struct fwctl_uctx *uctx);
    void (*close_uctx)(struct fwctl_uctx *uctx);
    void *(*info)(struct fwctl_uctx *uctx, size_t *length);
    void *(*fw_rpc)(struct fwctl_uctx *uctx, enum fwctl_rpc_scope scope, void *rpc_in, size_t in_len, size_t *out_len);
};

Members

device_type: The drivers assigned device_type number. This is uABI.
uctx_size: The size of the fwctl_uctx struct to allocate. The first bytes of this memory will be a fwctl_uctx. The driver can use the remaining bytes as its private memory.
open_uctx: Called when a file descriptor is opened before the uctx is ever used.
close_uctx: Called when the uctx is destroyed, usually when the FD is closed.
info: Implement FWCTL_INFO. Return a kmalloc() memory that is copied to out_device_data. On input length indicates the size of the user buffer on output it indicates the size of the memory. The driver can ignore length on input, the core code will handle everything.
fw_rpc: Implement FWCTL_RPC. Deliver rpc_in/in_len to the FW and return the response and set out_len. rpc_in can be returned as the response pointer. Otherwise the returned pointer is freed with kvfree().

Description

fwctl_unregister() will wait until all excuting ops are completed before it returns. Drivers should be mindful to not let their ops run for too long as it will block device hot unplug and module unloading.

struct fwctl_device¶: Per-driver registration struct

Definition:

struct fwctl_device {
    struct device dev;
};

Members

dev: The sysfs (class/fwctl/fwctlXX) device

Description

Each driver instance will have one of these structs with the driver private data following immediately after. This struct is refcounted, it is freed by calling fwctl_put().

fwctl_alloc_device¶

fwctl_alloc_device (parent, ops, drv_struct, member)

Allocate a fwctl

Parameters

parent: Physical device that provides the FW interface
ops: Driver ops to register
drv_struct: ‘struct driver_fwctl’ that holds the struct fwctl_device
member: Name of the struct fwctl_device in drv_struct

Description

This allocates and initializes the fwctl_device embedded in the drv_struct. Upon success the pointer must be freed via fwctl_put(). Returns a ‘drv_struct *’ on success, NULL on error.

struct fwctl_uctx¶: Per user FD context

Definition:

struct fwctl_uctx {
    struct fwctl_device *fwctl;
};

Members

fwctl: fwctl instance that owns the context

Description

Every FD opened by userspace will get a unique context allocation. Any driver private data will follow immediately after.

fwctl Driver design¶

In many cases a fwctl driver is going to be part of a larger cross-subsystem device possibly using the auxiliary_device mechanism. In that case several subsystems are going to be sharing the same device and FW interface layer so the device design must already provide for isolation and cooperation between kernel subsystems. fwctl should fit into that same model.

Part of the driver should include a description of how its scope restrictions and security model work. The driver and FW together must ensure that RPCs provided by user space are mapped to the appropriate scope. If the validation is done in the driver then the validation can read a ‘command effects’ report from the device, or hardwire the enforcement. If the validation is done in the FW, then the driver should pass the fwctl_rpc_scope to the FW along with the command.

The driver and FW must cooperate to ensure that either fwctl cannot allocate any FW resources, or any resources it does allocate are freed on FD closure. A driver primarily constructed around FW RPCs may find that its core PCI function and RPC layer belongs under fwctl with auxiliary devices connecting to other subsystems.

Each device type must be mindful of Linux’s philosophy for stable ABI. The FW RPC interface does not have to meet a strictly stable ABI, but it does need to meet an expectation that userspace tools that are deployed and in significant use don’t needlessly break. FW upgrade and kernel upgrade should keep widely deployed tooling working.

Development and debugging focused RPCs under more permissive scopes can have less stabilitiy if the tools using them are only run under exceptional circumstances and not for every day use of the device. Debugging tools may even require exact version matching as they may require something similar to DWARF debug information from the FW binary.

Security Response¶

The kernel remains the gatekeeper for this interface. If violations of the scopes, security or isolation principles are found, we have options to let devices fix them with a FW update, push a kernel patch to parse and block RPC commands or push a kernel patch to block entire firmware versions/devices.

While the kernel can always directly parse and restrict RPCs, it is expected that the existing kernel pattern of allowing drivers to delegate validation to FW to be a useful design.

Existing Similar Examples¶

The approach described in this document is not a new idea. Direct, or near direct device access has been offered by the kernel in different areas for decades. With more devices wanting to follow this design pattern it is becoming clear that it is not entirely well understood and, more importantly, the security considerations are not well defined or agreed upon.

Some examples:

HW RAID controllers. This includes RPCs to do things like compose drives into a RAID volume, configure RAID parameters, monitor the HW and more.

Baseboard managers. RPCs for configuring settings in the device and more

NVMe vendor command capsules. nvme-cli provides access to some monitoring functions that different products have defined, but more exist.

CXL also has a NVMe-like vendor command system.

DRM allows user space drivers to send commands to the device via kernel mediation

RDMA allows user space drivers to directly push commands to the device without kernel involvement

Various “raw” APIs, raw HID (SDL2), raw USB, NVMe Generic Interface, etc.

The first 4 are examples of areas that fwctl intends to cover. The latter three are examples of denied behavior as they fully overlap with the primary purpose of a kernel subsystem.

Some key lessons learned from these past efforts are the importance of having a common user space project to use as a pre-condition for obtaining a kernel driver. Developing good community around useful software in user space is key to getting companies to fund participation to enable their products.