Runtime code modification fails on arm

[Matthieu CASTET]

Papalagi Pakeha a écrit :
> Hi,
> 
> I've got a program that is stored partially encrypted on the
> filesystem and should decrypt itself in runtime after retrieving the
> key from the hardware.
> 
> Essentially the implementation puts some of the program functions into
> a separate ELF section (.cryptext) and then a helper script encrypts
> this section directly in the binary file. Offset and size is
> determined using "objdump -h".
> 
> When the program is started it finds the address of the encrypted
> function, its length and decrypts it back to the original valid
> instructions. This all works just fine on x86 but the same approach
> fails on ARM. There the decryptor can read the encrypted code, can
> write back the decrypted code, can verify that the code has been
> written but once the function is called it segfaults or dies on
> invalid instruction. To me it looks like the changed code is not
> picked up and the cpu still tries to run the old, encrypted one.
> 
> Why is this happening? What is so different between x86 and ARM in
> that field? I'm aware that my problem exhibits in userspace, not in
> the kernel. I'm sorry if it's way off topic here.
You need to flush the data cache and invalidate instruction one. For
that you can use __ARM_NR_cacheflush syscall.


Matthieu