mspbsldump/README.md

# MSP430FR BSL dumper

Tools to try to dump the MSP430FR BSL, mainly targetting the [MSP430FR5994
](https://www.ti.com/product/MSP430FR5994) (on an MSP-EXP430FR5994 devboard).

## The idea

The MSP430FR bootloader ('BSL') resides at `0x1000`. This memory cannot be
read, and user code can only jump to `0x1000` or `0x1002`, called the "Z-area",
to run certian functions of the BSL. Though, it is very likely that when the
CPU is running from inside this memory region, it can access this memory as
data, as that is often needed to store eg. structs, lookup tables, and so on.
Several other "execute-only" memory implementations function in a similar way,
such as the Nintendo GameBoy Advance and DS boot ROMS ("BIOS"es, citation
below), as well as in other systems analyzed by Schink and Obermaier,
publication also linked below.

The BSL (according to [the datasheet
](https://www.ti.com/lit/ug/slau550aa/slau550aa.pdf)) doesn't disable
interrupts. That means that, while the BSL is executing, it is possible to
interrupt this execution flow to
jump to code controlled by the user. An interrupt can inspect and modify
the registers of the BSL code at the time when the interrupt happened, as well
as the stack contents. Having a timer at the same frequency as the CPU, and
having it dump the register and stack contents after a certain interval increasing
by one cycle every iteration, it is possible to trace the instruction flow of
the CPU, as well as which registers and stack contents it is accessing, and how,
even though the code itself is not visible. Furthermore, the MSP430 CPU uses a
variable-length instruction set and instructions can use a variable amount of
cycles, therefore these traces can also be used to infer more infromation about
which instructions are executed, as the `pc` CPU register will never point to
the middle of an instruction, and will only advance to the next instruction
depending on how long the current instruction takes to execute.

Function epilogues typically first pop a number of values off the stack and
load these back into registers, and then return. By controlling the stack
pointer value, these can be used as a way to perform arbitrary reads. However,
as we are targetting nonwritable memory, an interrupt needs to happen before
the return occurs, otherwise CPU execution becomes very unpredictable.

You can find these epilogues by staring at many, many execution traces
(obtained from these timer interrupts) and thinking really hard (this is the
hard, time-consuming and labor-intensive part).

Alternatively, by timing an interrupt or a DMA transfer such that it happens
after a function is called but before it returns, it is possible to overwrite
the memory popped off the stack when an epilogue executes, thereby gaining
control of a few register values as well as the program counter. Then, CPU
execution can be redirected to another code snippet performing the memory read
before returning. With control over the address it reads from, this can be used
as an arbitrary read to read one word of the BSL, then return to use code to do
the next iteration.

The "using interrupts to figure out what execute-only code is doing" trick was
first (afaik) used by Martin Korth to find such a gadget inside the Nintendo DS
ARM7 boot ROM to read it out (and dump some keys), see [here
](http://problemkaputt.de/gbatek-bios-dumping.htm) and [here
](http://problemkaputt.de/gbatek-ds-memory-control-bios.htm), but is also
described in the academic literature, eg. [here
](https://www.usenix.org/system/files/woot19-paper_schink.pdf).

The "use DMA to get ROP" trick comes from [here
](https://hexkyz.blogspot.com/2021/11/je-ne-sais-quoi-falcons-over-horizon.html),
described near the end, the article is quite large.

## What has been implemented correctly

1. Memory in the BSL region cannot be read using data accesses from user code.
   Reads come back as `3f ff`, which decodes as an infinite loop.
1. Arbitrary code in the BSL region cannot be jumped to from user code, the
   CPU execution path has to go through the Z-area. Doing this will cause an
   infinite loop or a reset.

## Vulnerabilities of the BSL against a readout attack

1. When the CPU is executing the BSL, it can perform data accesses to other BSL
   areas. Thus, if an arbitrary read gadget is found, it can be used to dump
   the entire BSL region. This is the same issue as present in the Nintendo
   DS ARM7 boot ROM.
1. The routine at `0x1002` provides such a gadget, *as indicatd in SLAU550AA*.
1. The BSL execution is allowed to be interrutped, thus the instruction flow
   can be traced by dumping CPU register values throughout the BSL execution.
   This allows for finding arbitrary read gadgets.
1. Interrupts can also be used to change any register value while the BSL is
   executing, even at a specific point in time. This can be used to skip over
   certain instructions during analysis, for example.

## Vulnerabilities of the BSL against use as a source of ROP gadgets

1. The routine at `0x1002` returns quickly, *as indicatd in SLAU550AA*.
   Therefore, it can be used as an easy ROP entrypoint. This bypasses the "only
   call code from the Z-area" limitation.
1. Interrutps can be used to change return addresses etc., to jump to arbitrary
   locations inside the BSL.
1. Potentially, DMA transfers can also be used to change the stack contents,
   including return addresses, while the BSL is executing.

## Inaccurracies of the datasheets

1. The BSL clears all RAM from `0x1C00` to `0x3FC7`, not just `0x1C00` to
   `0x1FFF`.
1. The BSL also clears Tiny RAM and some "reserved" low addresses, from `6` to
   `0x1F`.
1. The BSL sets up Timer A, while the datasheet only mentions Timer B usage in
   *other* BSLs, and nothing about this one.

## What has not been checked

1. Pipelining: can code running at `0x0FFE` (or a similar address) access the
   BSL memory, (mis)using the possibility that the effective value of `pc`
   might differ from the executed address due to pipelining effects? (cf.
   MerryMage's GBA BIOS dump)
1. DMA: can a DMA transfer be used to change the stack contents during BSL
   execution? (Most likely, just like interrupts can, I simply haven't checked.)

## Hashes

This is the hash of the memory region `0x1000` to `0x17FF`, on an MSP430FR5994,
with BSL 00.08.35.B3:

| Hash function | value |
| ----- | ----- |
| MD5 | `4bb3bb753face80fffe1fef7a762884a` |
| SHA-1 | `1b4c13e006121a9b1c1ebcd4fbc6ec7c96cc017f` |
| SHA-256 | `e4d0d171013f847a357eebe5467bcd413ecb41dc01424b7e4ee636538d820766` |
| SHA-512 | `fed28a7e9643a551789075b79d9b04fa6e8cdca74d783c1c3830ece07e5c9141dda9532b3c442416a1ddab90d752e679c6918c0d5333ac6da9fd23ab6c33d1bb` |