TrustZone

Secure World, also known as TrustZone, Trusted Execution Environment, and (by Apple) Secure Enclave, is a sandboxed execution environment that has higher privileges that the normal Kernel. On the Vita, it seems that secure world has only a few tasks, which is facilitating communication with the F00D Processor, as well as SceGrab and SceSonyRegbus HW devices. This provides an additional layer of buffer between the ARM application processor and the CMeP security processor. In addition, after firmware 2.10, kernel process exceptions invoke a TrustZone call to initiate a kernel memory snapshot and encrypt it for use in a coredump.

Architecture

TrustZone files are stored in Kernel Boot Loader. TrustZone consists of secure kernel modules named "Tzs" which are loaded and started by SKBL. Once that Tzs modules have been started, and that SKBL has finished its work, SKBL starts NSKBL. Tzs modules are actually modules because they just wait for their exported functions to be called. The special thing is that Tzs libraries exports are only accessible to Tzs, so non-secure kernel modules cannot import them. For that reason, Tzs SceExcpmgr registers SMCs.

Security

The main security of Secure World is the same security that prevents kernel access from usermode which is that the secure kernel code is completely proprietary and cannot be seen in Non-secure World. Without access to the code, it is hard to develop a targeted attack on Secure World. However, once a Secure World memory dump is achieved through a memory leak exploit, one can see that the ARM secure kernel lacks most of the security features found in the non-secure kernel. This is likely because the ARM secure kernel is only to provide an extra layer of protection to prevent unauthorized access to CMeP and does not function as it does on other TrustZone enabled devices (the iPhone for example uses it to store fingerprint data). Most of the above-kernel security will be found in that processor.

Tzs Region Protection

Since Tzs resides in the same DRAM as NS, it can be referenced/read from NS.

To prevent that, SCE adds Tzs region to bypass list in Xbar->MainXbar->LPDDR0_config just before SKBL jumps to NS so that only secure state can access Tzs.

So after the bypass is added, accessing Tzs from NS with memalloc with pbase, for example, will cause a DABT.

Secure Devices

See Physical Memory for a list of known devices that can only be accessed in the Secure World. The DRAM region 0x40000000 for 2MB (3MB prior to FW 3.50) can only be accessed in Secure World. This is where the secure bootloader and kernel are loaded to. CMeP can only be accessed in Secure World and only Secure World can handle interrupts from that processor.

SCR

After the boot initialization, the SCR is set to 0x00000004 which means FIQs are handled in Secure World.