SGX - Self-modifying Code

[John_C_3]

Is self-modifying code allowed in SGX enclaves?  I created a simple example that just calls a function stored in a data buffer.  I changed the properties for the enclave DLL so that data is also executable.  It worked when I compiled the project in simulation mode, but it crashes in hardware mode.

[Matthew_H_Intel]

How are you calling the function?  call/jump etc.

[John_C_3]

I cast the data buffer as a function pointer and then call the function pointer.  I don't use any inline assembly.  To get the bytes for the data buffer, I compiled a project with a simple function (in this case just returning the sum of two parameters), and extracted the bytes of the function from the exe.

[Matthew_H_Intel]

I don't see any issues with that, off the top of my head but I'll confer with some experts and report back.  Meanwhile, can you post the disassembly for your function?  From your description it sounds like it should be small.

[John_C_3]

Yes, the disassembly is not too long.  Here it is:

55                   	push   rbp
8b ec                	mov    ebp,esp
81 ec c0 00 00 00    	sub    esp,0xc0
53                   	push   rbx
56                   	push   rsi
57                   	push   rdi
8d bd 40 ff ff ff    	lea    edi,[rbp-0xc0]
b9 30 00 00 00       	mov    ecx,0x30
b8 cc cc cc cc       	mov    eax,0xcccccccc
f3 ab                	rep stos DWORD PTR es:[rdi],eax
8b 45 08             	mov    eax,DWORD PTR [rbp+0x8]
03 45 0c             	add    eax,DWORD PTR [rbp+0xc]
5f                   	pop    rdi
5e                   	pop    rsi
5b                   	pop    rbx
8b e5                	mov    esp,ebp
5d                   	pop    rbp
c3                   	ret