.. _boot-chain-label:
=======
Booting
=======

After exploitation the goal is to fully boot the device.

Current boot chain:

.. figure:: images/boot_chain.drawio.svg
   :align: center

   Boot chain

As done by Frederic, the bootrom can be dumped using his provided scripts, and can the be split into different boots:

.. code-block:: bash

    ./exynos-usbdl e payloads/Exynos8890_dump_bootrom.bin dumped_sboot.bin
    scripts/split-sboot-8890.sh dumped_sboot.bin

debugger
========
Some other information about the debugger and it's current state.

I relocated the debugger to ``0x20c0000`` to prevent overwriting it.

.. code-block:: python 

    self.cd.arch_dbg.state.auto_sync = False
    self.cd.arch_dbg.state.auto_sync_special = False
    self.cd.arch_dbg.state.print_ctx()
    
    def relocate_debugger():
        # Seems to be cleared upon cache clearing??
        debugger_reloc = open("/home/eljakim/Source/gupje/source/bin/samsung_s7/reloc_debugger.bin", "rb").read()
        self.cd.memwrite_region(0x020c0000, debugger_reloc)
        self.usb_write(b"FLSH") # Flush cache
        self.cd.restore_stack_and_jump(0x020c0000)
        assert self.usb_read(0x200) == b"GiAs", "Failed to relocate debugger"
        self.cd.relocate_debugger(0x020c7000, 0x020c0000, 0x020c4000)
    relocate_debugger()




bl1
===
BL1 needs to be authenticated.
Loads at address ``0x02024000`` and contains some form of header (ramdump).
There seems to be a samsung header format, where the first 4 bytes define the entry point of the binary. 
In this case this entry is ``+0x10`` so we jump to ``0x02024010``.

.. code-block:: python

    # Try loading bl1
    bl1 = open("../S7/bl1.bin", "rb").read()
    self.cd.memwrite_region(0x02021800, bl1)
    # self.usb_write(b"FLSH")
    AUTH_BL1 = 0x00012848
    def auth_bl1(lr=0x2069000):
        # Load the firmware
        self.cd.arch_dbg.state.W0 = 1
        self.cd.arch_dbg.state.X1 = 1
        self.cd.arch_dbg.state.LR = lr #jump back to debugger when finished
        self.cd.restore_stack_and_jump(AUTH_BL1)
        assert self.usb_read(0x200) == b"GiAs", "Failed to jump back to debugger"
        assert self.cd.arch_dbg.state.X0 == 0, "auth_bl1 returned with error!"
        
    auth_bl1(0x020c0000)

After authentication the bootROM jumps to it, we can execute this function with the debugger.

.. code-block:: python

    self.cd.memwrite_region(0x02020f60, p32(0x020c0000)) 
        BOOT_BL1 = 0x00019310
        def jump_bl1(lr):
            self.cd.arch_dbg.state.LR = lr
            self.cd.restore_stack_and_jump(BOOT_BL1)
        
        jump_bl1(0x020c0000)

    jump_fwbl1()

BL1 is laoded at the download buffer and self copies to ``0x02024000`` and resumes execution there(``0x02024010``).

However, this does not result in a jump back to the debugger. But the ROM still receives data from the host

TODO TODO TODO 
The reason for this is the following code in bl1:

.. code-block:: c

    iVar3 = FUN_02024320();
    if (iVar3 == 1) {
        (*(code *)(ulong)uRam0000000002020108)(0,1);
    }
    
This code uses a predefined ROM function(I was looking for it) and jumps back to that function when it's done. 
This function is at address ``0x020200e8``, looking in our IMEM dump we can see where in the ROM this points to:

.. code-block:: c

                             DAT_02020108                                    XREF[2]:     FUN_00001708:000018b4(W), 
                                                                                          FUN_02021970:02021a40(R)  
        02020108 90 57 00 00     undefined4 00005790h

Replacing this function with our debugger makes us jump back:

.. code-block:: python

    # Overwrite jump back
        self.cd.memwrite_region(0x02020108, p32(0x2069000))
        self.cd.memwrite_region(0x020200e8, p32(0x2069000))
            
        def jump_bl1():
            self.cd.arch_dbg.state.LR = 0x2069000
            self.cd.restore_stack_and_jump(0x02024010)
            # self.cd.restore_stack_and_jump(0x02021810)
            
        bl1 = open("../S7/bl1.bin", "rb").read()
        self.cd.memwrite_region(0x02024000, bl1)
        self.usb_write(b"FLSH")
        
        # auth_bl1()
        jump_bl1()
        assert self.usb_read(0x200) == b"GiAs", "not jumped back to debugger?"
        self.cd.arch_dbg.state.print_ctx()

        root | DEBUG | 
            X0 : 0xc00000 | X1 : 0x2069000 | X2 : 0x0 | X3 : 0x2023114 | X4 : 0x4 | X5 : 0x0 | X6 : 0x0 |
            X7 : 0x136c0008 | X8 : 0x2069000 | X9 : 0x0 | X10 : 0x2070000 | X11 : 0x0 | X12 : 0x0 | X13 : 0x0 |
            X14 : 0xf | X15 : 0x206d000 | X16 : 0x9 | X17 : 0x0 | X18 : 0x1 | X19 : 0x20200e8 | X20 : 0x0 |
            X21 : 0x80000000 | X22 : 0x0 | X23 : 0x0 | X24 : 0x0 | X25 : 0x0 | X26 : 0x0 | X27 : 0x1 |
            X28 : 0x0 | X29 : 0x2020ed8 | LR/X30 : 0x202419c | SP/X31 : 0x2020ec0

However this does not fully run bl1, so we will have to dig a bit deeper to see the puropose and when to jump back to the debugger.

purpose
-------
bl1 interacts with several pheriperals, from the DTB these are:

.. code-block:: dtsi

    /* FSYS0 */
	pinctrl_5: pinctrl@10E60000 {
		compatible = "samsung,exynos8890-pinctrl";
		reg = <0x0 0x10E60000 0x1000>;
		interrupts = <0 212 0>;
	};

	/* FSYS1 */
	pinctrl_6: pinctrl@15690000 {
		compatible = "samsung,exynos8890-pinctrl";
		reg = <0x0 0x15690000 0x1000>;
		interrupts = <0 202 0>;
	};

    /* PERIC1 */
	pinctrl_9: pinctrl@14CC0000 {
		compatible = "samsung,exynos8890-pinctrl";
		reg = <0x0 0x14CC0000 0x1000>;
		interrupts = <0 460 0>;
	};

    pmu_system_controller: system-controller@105C0000 {
		compatible = "samsung,exynos8890-pmu", "syscon";
		reg = <0x0 0x105C0000 0x10000>;
	};

    rtc@10070000 {
		compatible = "samsung,s3c6410-rtc";
		reg = <0x0 0x10070000 0x100>;
		interrupts = <0 73 0>, <0 74 0>;
		clocks = <&clock 157>;
		clock-names = "gate_rtc";
	};


BL31
----

Setups EL3 stuff, probably in preperation of loading trustzone