[U-Boot] Sharing code between U-Boot and its secure monitor

[Stephen Warren]

On 06/24/2018 12:17 AM, Alexander Graf wrote:
> 
> 
>> Am 22.06.2018 um 21:24 schrieb Stephen Warren <swarren at wwwdotorg.org>:
>>
>> I am writing more secure monitor (PSCI) support for Jetson TK1. In particular, this code will support resume from the LP0 system suspend state, which entails signing a block of code for the boot ROM to validate during resume. The signing code uses lib/aes.c. This code is linked into U-Boot in the "main" part of U-Boot, rather than into the secure section that contains the monitor code. The monitor should only call code within the monitor section, not within the main part of U-Boot (in general, the monitor continues to run after U-Boot has been replaced in RAM, so cannot call code in U-Boot since it may no longer exist, and even if it does, that code is not in the secure DRAM carve-out, and hence it's not secure to call it). So, I need to duplicate the lib/aes.c object code into the monitor. The question is: what's the best way to do that.
>>
>> So far, here's what I implemented:
>>
>> a) Edit lib/aes.c to mark each function with an optional attribute which will force the object code into the secure section when compiled for the monitor:
>>
>> -static const u8 sbox[256] = {
>> +static const u8 mon_rodata sbox[256] = {
>>
>> -void aes_cbc_decrypt_blocks(...
>> +void mon_text aes_cbc_decrypt_blocks(...
>>
>> ... where mon_text evaluates to:
>>
>> +#define mon_rodata
>> +#define mon_text
>>
>> or:
>>
>> +#define mon_data    __attribute__((section("._secure.data")))
>> +#define mon_rodata    __attribute__((section("._secure.rodata")))
>> +#define mon_text    __attribute__((section("._secure.text")))
> 
> Please check my recent fix to rename the efi sections. Gcc may under some conditions generate implicit symbols, such as rodata constants. You can only catch them if your text section for the function starts with .text.
> 
>>
>> b) Since the main U-Boot and the monitor code are part of the same ELF file, the same symbol name cannot exist in both. So, we must play similar games in order to rename the symbols:
>>
>> -void aes_cbc_decrypt_blocks(...
>> +void mon_text MON_SYM(aes_cbc_decrypt_blocks)(...
>>
>> (all call sites have to be updated similarly)
>>
>> ... where MON_SYM(x) is either:
>>
>> +#define MON_SYM(x) x
>>
>> or:
>>
>> +#define MON_SYM(x) mon_##x
>>
>> c) In the monitor, create a file mon_aes.c that sets up all the macros mentioned above, then #includes the main lib/aes.c. Add this file to a Makefile.
>>
>> +#include "mon_section.h"
>> +#include "../../../../../lib/aes.c"
>>
>> This is all rather nasty and invasive, especially when you consider more widely used utility functions such as malloc(), printf(), udelay(), etc.. Instead, I wonder if we can:
>>
>> a) Link the monitor to an ELF file and extract a binary. We won't need any special section or symbol name logic here, since we can assume that all of .text/.data are part of the monitor. Simple and non-invasive!
>>
>> b) Link the LP0 resume code to an ELF file and extract a binary. (It's nice to separate this block of code since it runs on a different CPU to the main U-Boot or monitor, and hence gets compiled with different compiler flags).
>>
>> c) Include the binaries from (a) and (b) above in the main U-Boot ELF file or binary somehow; perhaps use binman to allow the main U-Boot to know where those merged binaries end up in memory.
>>
>> In a slightly different context, I've talked to Simon in the past about building many separate binaries from U-Boot's source and chaining between them and merging them together and he objected to that approach. However, I wonder if this new requirement changes anything?
>>
>> Thanks for any thoughts.
> 
> This looks quite similar to efi runtime services requirements to me. Maybe we can share code.
> 
> The way those work is that we mark all functions and data required in special sections too. We also include all relocations inside a special section as well though.
> 
> If you follow the same scheme, you could simply clone parts of U-Boot at runtime with different relocations to either main U-Boot or mon. During that relocation you could also find out if there is any relication that is unreachable from mon code. That could trigger a warning that CI should be able to quickly find.

So we'd have the following chunks of code (lets say sections):

1) U-Boot only
2) Monitor (or UEFI) only
3) Shared code between 1 and 2.

... then memcpy (3) to two places, one for (1) and one for (2)?

On the surface that seems plausible, but what happens if we have the 
following main chunks of code:

1) U-Boot
2) Monitor
3) UEFI
4) Something else.

Now we either have a relatively large and bloated dumping ground for all 
common code:

5) Common code, which gets copied 4 other places, and contains many 
functions some of the copy targets don't need.

... or many combinations:

5) Code shared between 1, 2, 3, 4
6) Code shared between 1, 2, 3
7) Code shared between 1, 2, 4
8) Code shared between 1, 3, 4
...

Or a per-function section and the relocator iterates over each 
per-function section separately, and works out which of 1..4 it gets 
copied into.

None of those options seem very tractable once you get more than a 
couple of potential copy destinations.

Building each binary separately means the linker works out which 
functions to add to each binary at compile time, so saves complexity or 
manual management of function sections, and saves runtime work.