Dynamic C code execution: memory references

Question

tl;dr : I'm trying to execute dynamically some code from another snippet. But I am stuck with handling memory reference (e.g. mov 40200b, %rdi): can I patch my code or the snippet running code so that 0x40200b is resolved correctly (as the offset 200b from the code)?

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To generate the code to be executed dynamically I start from a (kernel) object and I resolve the references using ld.

#!/usr/bin/python
import os, subprocess
if os.geteuid() != 0:
    print('Run this as root')
    exit(-1)
with open("/proc/kallsyms","r") as f:
    out=f.read()
sym= subprocess.Popen( ['nm', 'ebbchar.ko', '-u' ,'--demangle', '-fposix'],stdout=subprocess.PIPE) 
v=''
for sym in sym.stdout:
    s = " "+ sym.split()[0]+ "\n"
    off = out.find(s)
    v += "--defsym "+s.strip() + "=0x" +out[off-18:off -2]+" "
print(v)
os.system("ld ebbchar.ko "+ v +"-o ebbchar.bin");

I then transmit the code to be executed with through a mmaped file

int fd = open(argv[1], O_RDWR | O_SYNC);
address1 = mmap(NULL, page_size, PROT_WRITE|PROT_READ , MAP_SHARED, fd, 0);
int in=open(argv[2],O_RDONLY);
sz= read(in, buf+8,BUFFER_SIZE-8);
uint64_t entrypoint=atol(argv[3]);
*((uint64_t*)buf)=entrypoint;
write(fd, buf, min(sz+8, (size_t) BUFFER_SIZE));

I execute code dynamycally with this code

struct mmap_info *info;
copy_from_user((void*)(&info->offset),buf,8);
copy_from_user(info->data, buf+8, sz-8);
unsigned long (*func)(void)  func= (void*) (info->data + info->offset);
int ret= func();

This approch work for code that don't access memory such as "\x55\x48\x89\xe5\xc7\x45\xf8\x02\x00\x00\x00\xc7\x45\xfc\x03\x00\x00\x00\x8b\x55\xf8\x8b\x45\xfc\x01\xd0\x5d\xc3" but I have problems when memory is involved.

See example below.

Let's assume i wan't execute dynamically the function vm_close. Objdump -d -S returns:

0000000000401017 <vm_close>:
{
  401017:   e8 e4 07 40 81          callq  ffffffff81801800 <__fentry__>
    printk(KERN_INFO "vm_close");
  40101c:   48 c7 c7 0b 20 40 00    mov    $0x40200b,%rdi
  401023:   e9 b6 63 ce 80          jmpq   ffffffff810e73de <printk>

At execution, my function pointer points to the right code:

(gdb) x/12x $rip
0xffffc90000c0601c:     0x48    0xc7    0xc7    0x0b    0x20    0x40    0x00    0xe9
0xffffc90000c06024:     0xb6    0x63    0xce    0x80
(gdb) x/2i $rip
=> 0xffffc90000c0601c:  mov    $0x40200b,%rdi
   0xffffc90000c06023:  jmpq   0xffffc8ff818ec3de

BUT, this code will fail since:

1) In my context $0x40200b points at the physical address $0x40200b, and not offset 200b from the beginning of the code.

2) I don't understand why but the address displayed there is actually different from the correct one (0xffffc8ff818ec3de != ffffffff810e73de) so it won't point on my symbol and will crash.

Is there a way to solve my 2 issues?

Also, I had trouble to find good documentation related to my issue (low-level memory resolution), if you could give me some, that would really help me.

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Edit: Since I run the code in the kernel I cannot simply compile the code with -fPIC or -fpie which is not allowed by gcc (cc1: error: code model kernel does not support PIC mode)

Edit 24/09: According to @Peter Cordes comment, I recompiled it adding mcmodel=small -fpie -mno-red-zone -mnosse to the Makefile (/lib/modules/$(uname -r)fixed/build/Makefile) This is better than in the original version since the generated code before linking is now:

0000000000000018 <vm_close>:
{
  18:   ff 15 00 00 00 00       callq  *0x0(%rip)        # 1e <vm_close+0x6>
    printk(KERN_INFO "vm_close");
  1e:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # 25 <vm_close+0xd>
  25:   e8 00 00 00 00          callq  2a <vm_close+0x12>
}
  2a:   c3                      retq   So thanks to rip-relative addressing 

Thus I’m now able to access the other variables on my script…

Thus, after linking I can successfully access my variable embedded within the buffer.

40101e: 48 8d 3d e6 0f 00 00    lea    0xfe6(%rip),%rdi        # 40200b

Still, one problem remains:

The symbol I want to access (printk) and my executable buffer are in different address spaces, for exemple:

printk=0xffffffff810e73de:
Executable_buffer=0xffffc9000099d000

But in my callq to printk, I have only 32 bits to write the address to call as an offset from $rip since there is no .got section in the kernel. This means that printk has to be located within [$rip-2GO, $rip+2GO]. But this is not the case there.

Do I have a way to access the printk address although they are located more than 2GO away from my buffer (I tried to used mcmodel=medium but I haven't seen any difference in the generated code), for instance by modifying gcc options so that the binary actually have a .got section?

Or is there a reliable way to force my executable and potentially-too large-for-kmalloc buffer to be allocated in the [0xffffffff00000000 ; 0xffffffffffffffff] range? (I currently use __vmalloc(BUFFER_SIZE, GFP_KERNEL, PAGE_KERNEL_EXEC); )

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Edit 27/09: I succedded in allocationg my buffer in the [0xffffffff00000000 ; 0xffffffffffffffff] range using the non exported __vmalloc_node_range function as a (dirty) hack.

IMPORTED(__vmalloc_node_range)(BUFFER_SIZE, MODULE_ALIGN,
                MODULES_VADDR + get_module_load_offset(),
                MODULES_END, GFP_KERNEL,
                PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
                __builtin_return_address(0));

Then, when I know the address of my executable buffer and the address of the kernel symbols (by parsing /proc/kallsyms), I can patch my binary using ld’s option --defsym symbol=relative_address where relative_address = symbol_address - buffer_offset .

Despite being extremely dirt, this approach actually works.

But I need to relink my binary each time I execute it since the buffer may (and will) be allocated at a different address. To solve this issue, I think the best way would be to build my executable as a real position independent executable so that I can just patch the global offset table and not fully relink the module.

But with the options provided there I got a rip-relative address but no got/plt. So I'd like to find a way to build my module as a proper PIE.

This post is getting huge, messy and we are deviating from the original question. Thus, I opened a new simplified post there. If I get interesting answers, I'll edit this post to explain them.

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Note: For the sake of simplicity, safety tests are not displayed there

Note 2: I am perfectly aware that my PoC is very unusual and can be a bad practice but I'd like to do it anyway.