UserSendCDB fails due to permissions

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I created a custom class that inherits from IOUserSCSIPeripheralDeviceType00 in the DriverKit SCSIPeripheralsDriverKit framework. When I attempted to send a vendor-specific command to a USB storage device using the UserSendCDB function of this class instance, the function returned the error:

kIOReturnNotPrivileged  (iokit_common_err(0x2c1))  // privilege violation

However, when using UserSendCDB in the same way to issue standard SCSI commands such as INQUIRY or Test Unit Ready, no error occurred and the returned sense data was valid.

Why is UserSendCDB able to send standard SCSI commands successfully, but vendor-specific commands return kIOReturnNotPrivileged? Is there any required entitlement, DriverKit capability, or implementation detail needed to allow vendor-specific CDBs?

Below are the entitlements of my DriverKit extension:

<dict>
    <key>com.apple.developer.driverkit.transport.usb</key>
    <array>
        <dict>
            <key>idVendor</key>
            <integer>[number of vendorid]</integer>
        </dict>
    </array>
    <key>com.apple.developer.driverkit</key>
    <true/>
    <key>com.apple.developer.driverkit.allow-any-userclient-access</key>
    <true/>
    <key>com.apple.developer.driverkit.allow-third-party-userclients</key>
    <true/>
    <key>com.apple.developer.driverkit.communicates-with-drivers</key>
    <true/>
    <key>com.apple.developer.driverkit.family.scsicontroller</key>
    <true/>
</dict>

If there is any additional configuration or requirement to enable vendor-specific SCSI commands, I would appreciate your guidance.

Environment: macOS15.6 M2 MacBook Pro

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Answer 1

DTS Engineer OP

Apple

21h

However, when using UserSendCDB in the same way to issue standard SCSI commands such as INQUIRY or Test Unit Ready, no error occurred and the returned sense data was valid.

You're allowed to send some commands (basically, anything that's inherently non-disruptive) at any time.

Why is UserSendCDB able to send standard SCSI commands successfully, but vendor-specific commands return kIOReturnNotPrivileged? Is there any required entitlement, DriverKit capability, or implementation detail needed to allow vendor-specific CDBs?

Did you call UserSuspendServices() first? That's required to send vendor specific commands?

Below are the entitlements of my DriverKit extension:

Just to make sure this is clear and because entitlements tend to be blamed for lots of issues, no, this is not an entitlement issue.

__
Kevin Elliott
DTS Engineer, CoreOS/Hardware

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