boltd is the thunderbolt device manager daemon. Its goal is to enable the secure and convenient use of thunderbolt devices by using the security features of modern thunderbolt controllers. It provides the org.freedesktop.bolt name on the system bus. boltd is autostarted via systemd/udev if a thunderbolt device is connected.
The thunderbolt I/O technology works by bridging PCIe between the controllers on each end of the connection, which in turn means that devices connected via Thunderbolt are ultimately connected via PCIe. Therefore thunderbolt can achieve very high connection speeds, fast enough to even drive external graphics cards. The downside is that it also makes certain attacks possible. To mitigate these security problems, the latest version --- known as Thunderbolt 3 --- supports different security levels:
The primary task of boltd is to authorize thunderbolt peripherals if the security level is either user or secure. It provides a D-Bus API to list devices, enroll them (authorize and store them in the local database) and forget them again (remove previously enrolled devices). It also emits signals if new devices are connected (or removed). During enrollment devices can be set to be automatically authorized as soon as they are connected. A command line tool, called boltctl(1), can be used to control the daemon and perform all the above mentioned tasks.
The pre-boot access control list (BootACL) feature is active when supported by the firmware and when boltd is running on a new enough Linux kernel (>= 4.17). The BootACL is a list of UUIDs, that can be written to the thunderbolt controller. If enabled in the BIOS, all devices in that list will be authorized by the firmware during pre-boot, which means these devices can be used in the BIOS setup and also during Linux early boot. NB: no device verification is done, even when the security level is set to secure mode in the BIOS, i.e. the maximal effective security level for devices in the BootACL is only user. If BootACL support is present, all new devices will be automatically added. Devices that are forgotten (removed from boltd) will also be removed from the BootACL. When a controller is offline, changes to the BootACL will be written to a journal and synchronized back when the controller is online again.
IOMMU support: if the hardware and firmware support using the input-output memory management unit (IOMMU) to restrict direct memory access to certain safe regions, boltd will detect that feature and change its behavior: As long as iommu support is active, as inidcated by the iommu_dma_protection sysfs attribute of the domain controller, new devices will be automatically enrolled with the iommu policy and existing devices with iommu (or auto) policy will be automatically authorized by boltd without any user interaction. When iommu is not active, devices that were enrolled with the iommu policy will not be authorized automatically. The status of iommu support can be inspected by using boltctl domains.
Written by Christian Kellner <email@example.com>.