82 lines
2.5 KiB
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82 lines
2.5 KiB
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===================
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Exynos BootROM 8890
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====================
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The Exynos 8890 BootROM is a small piece of code that runs on the Exynos SoC at boot time.
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It is responsible for initializing the hardware and loading the first stage bootloader from storage.
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The BootROM is stored in a read-only memory and cannot be modified.
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Protections
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-----------
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There are no stack canaries or guard pages, and no ASLR. Meaning there are almost no protections in place.
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Samsung Firmware
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----------------
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Samsung releases firmware files for their devices. These files contain the bootloader, modem, and other firmware files.
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To see how the ROM works we are interested in the sboot firmware, which contains multiple stages of the bootloader.
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To extract the sboot.bin file from a samsung firmware file:
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.. code-block:: bash
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$ unzip -p firmware.zip 'BL_*.tar.md5' | tar -Oxf - 'sboot.bin.lz4' | lz4 -d - sboot.bin
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Memory Layout
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-------------
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TODO make memory layout of ROM, IMEM and some devices
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.. figure:: images/memory_layout.drawio.svg
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The memory layout of the Exynos 8890
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Download protocol
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=================
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When the ROM is unable to boot from the internal storage, it enters ``Exynos Recovery Mode``.
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In this mode the bootROM accepts data over USB.
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There is little functionality other than receiving data, meaning almost no additional attack surface except for the download protocol.
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The Exynos BootROM uses a custom protocol to download a bootable image over USB.
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This image is verified and executed by the BootROM. Unauthorized images are rejected. (TODO verify and document)
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dldata
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------
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.. figure:: images/dl_packet.drawio.svg
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The dldata packet is used to send data to the BootROM.
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.. info::
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This protocol remains *mostly* the same for newer Exynos SoCs.
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Bug 1(Integer underflow)
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------------------------
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@TODO better explain frederick's bug. @JOHER
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Bug 2
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-----
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.. caution::
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Might be a 0/N-day if exploitable
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@ELHER
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There is a bug(unpatched?) in receiving the last packet of the usb image:
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.. figure:: images/underflow_bug.png
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The bug is an integer underflow in the calculation of the remaining size of the image.
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DWC3
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====
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The Exynos 8890 uses the Synopsys DesignWare USB 3.0 controller.
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Much of the code is shared with the DWC3 driver in the Linux kernel, except that the ROM does not do any scheduling and a lot of features have been removed(OTG handling, etc).
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Gupje
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-----
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In order to run the debugger, a small amount of the bootROM was reversed in order to implement send/recv functionality.
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