C Program to test shellcode

shellcode.c

#include<stdio.h>
#include<string.h>

unsigned char code[] = \
"\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80";

main()
{

  printf("Shellcode Length:  %d\n", strlen(code));

	int (*ret)() = (int(*)())code;

	ret();

}

At least on my x64 the memory page is not executable (x86 only has r/w, x64 has r/w/x, thats why it always works on x86 and the problem does not occur there)

Not exactly. x86 also has r/w/x but the ELF loading code in the kernel (for x86 only) treats all readable memory as executable IF the stack was marked as executable. Which it is if either PT_GNU_STACK program header is missing in the ELF or if it is present and has the executable flag set. Which is why the -z execstack was important.
See:

• https://elixir.bootlin.com/linux/v6.1.9/source/fs/binfmt_elf.c#L933
• https://elixir.bootlin.com/linux/v6.1.9/source/fs/binfmt_elf.c#L1009
• https://elixir.bootlin.com/linux/v6.1.9/source/arch/x86/include/asm/elf.h#L293

Shameless self promotion: I made a tool for shellcode execution which you may want to use: https://github.com/RobertLarsen/RunShellcode

Here is another example to run x86 shellcode on x64 machine but specify the memory address where you want to load your shellcode:

# include <stdio.h>
# include <string.h>
# include <unistd.h>
# include <sys/mman.h>

# define EXEC_MEM ((void *) 0x80000000)

char shellcode[] = "{write your shellcode here}";

int main() {{
    mmap(EXEC_MEM, 0x1000, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_FIXED | MAP_PRIVATE, -1, 0);
    memcpy(EXEC_MEM, (void *)shellcode, strlen(shellcode)+1);
    (*(int (*)())EXEC_MEM)();
    return 0;
}

Just put code in stack by initializing code as local variable

#include <stdio.h>
#include <string.h>

int main(){
    char code[] = "\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80";
    printf("Shellcode length: %d\n", strlen(code));
    int (*ret)() = (int(*)())code;
    return ret();
}

• gcc -fno-stack-protector -z execstack -m32 shellcode.c -o shellcode

For some reason this God forsaken shellcode works when all the other ones I found online/generated refused to do so. Thank you from the bottom of my heart I spent way too much time on this