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The IOUserClient[11] is a subclass of IOService that serves as a secure
bridge between user-space applications and kernel I/O service objects.
It does not interact with hardware directly but provides a controlled
interface for safe communication with kernel services. IOUserClient objects
serve as security gatekeepers, running in kernel space and handling
requests from unprivileged user-space applications. They validate input,
enforce access controls, and sanitize data before passing it to services.
The core logic is implemented in newUserClient[12] functions.
=================================
--[ 1.2.1 - Multiple User Clients per Service
============================================================================
A single Service can have multiple User Clients registered. They can
expose different interfaces to the same underlying Service. Each User
Client type has a unique numeric identifier (uint32_t). Applications
specify which User Client type they want when connecting with
IOServiceOpen()[13]. Type 0 is typically the default/primary interface.
================
--[ 1.2.2 - External Methods
============================================================================
The externalMethod[14] within the User Client handles incoming
IOConnectCallMethod[15] requests from the user-space app. It validates that
the selector is within bounds and argument sizes. Based on the selector
value, it routes them to appropriate handler functions. These are the final
endpoints where core logic and most of the vulnerabilities lie.
==================
--[ 1.2.3 - Dispatch Mechanism
============================================================================
At WWDC22, the validation portion of the external method was moved to a
new "2022" dispatchExternalMethod, which serves as a wrapper around the
method array (sIOExternalMethodArray):
````````````````````````````````````````````````````````````````````````````
IOReturn AppleJPEGDriverUserClient::externalMethod(
IOUserClient* userClient, // this user client instance
uint32_t selector, // method id from user space
IOExternalMethodArguments* arguments // I/O parameters from user space
)
{
return IOUserClient2022::dispatchExternalMethod(
userClient, // the user client object
selector, // which method to call (0-9)
arguments, // parameters from user space
&sIOExternalMethodArray, // dispatch table with 10 methods
10, // number of methods in table
userClient, // target object for method calls
0 // additional flags/options
);
}
````````````````````````````````````````````````````````````````````````````
The dispatchExternalMethod()[16] validates the selector is within bounds,
calculates the dispatch table entry at methodArray[selector], checks
arguments sizes (I/O scalars and structs), optionally validates
entitlements for privileged operations, and finally, call the target
handler function if all checks pass.
==========================
--[ 1.2.4 - getTargetAndMethodForIndex
============================================================================
Although many UserClient::externalMethods were rewritten to include
IOUserClient2022::dispatchExternalMethods, there is still a significant
amount of code that follows the old method[17]. There are also
getTargetAndMethodForIndex[18]. Drivers using the old way are more prone to
misuse or missing validation.
==============
--[ 1.2.5 - Access Control
============================================================================
The same selector can mean different things in different User Clients:
````````````````````````````````````````````````````````````````````````````
IOService "MyDevice"
|- IOUserClient Type 0 (standard interface)
| |- Selector 0: GetStatus
| `- Selector 1: SetConfig
`- IOUserClient Type 1 (admin interface)
|- Selector 0: FactoryReset
`- Selector 1: UpdateFirmware
````````````````````````````````````````````````````````````````````````````
This design provides both functional separation and security boundaries -
unprivileged apps receive limited user clients, while privileged ones
receive full-featured ones. The entitlements[19] embedded in the
application's code signature[20] define access to these interfaces.
==============
--[ 1.3 - User Space App
============================================================================
User-space applications can be sandboxed, restricting their access to
IOKit even if they possess the necessary entitlements. Sandboxed apps are
generally restricted from performing sensitive operations, such as opening
hardware service connections or modifying properties. On macOS Sequoia, the
Sandbox Operations affecting IOKit access include[21]:
- "iokit*"
- "iokit-get-properties"
- "iokit-issue-extension"
- "iokit-open*"
- "iokit-open-user-client"
- "iokit-open-service"
- "iokit-set-properties"