A 64 bit CPU (amd64) supports 32 bit Intel instructions in the compatibility mode. Also, a 64 bit Linux installation allows running ELFs containing 32 bit instructions if the ELF header says it is a 32 bit executable.
I was wondering if it is possible to put some assembly instructions inside the ELF which switches the CPU to 32 bit compatibility mode in the middle of a program (and then later back again)? If these assembly instructions are not permitted by the kernel, is there perhaps some way we can get the kernel to switch an already running process to 32 bit?
This is mainly a question out of curiosity since I cannot really see any use-cases for it.
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Answer
Switching between long mode and compatibility mode is done by changing CS. User mode code cannot modify the descriptor table, but it can perform a far jump or far call to a code segment that is already present in the descriptor table. I think that in Linux (for example) the required compatibility mode descriptor is present.
Here is sample code for Linux (Ubuntu). Build with
$ gcc -no-pie switch_mode.c switch_cs.s
switch_mode.c:
#include <stdlib.h> #include <stdio.h> #include <stdbool.h> extern bool switch_cs(int cs, bool (*f)()); extern bool check_mode(); int main(int argc, char **argv) { int cs = 0x23; if (argc > 1) cs = strtoull(argv[1], 0, 16); printf("switch to CS=%02xn", cs); bool r = switch_cs(cs, check_mode); if (r) printf("cs=%02x: 64-bit moden", cs); else printf("cs=%02x: 32-bit moden", cs); return 0; }
switch_cs.s:
.intel_syntax noprefix .code64 .text .globl switch_cs switch_cs: push rbx push rbp mov rbp, rsp sub rsp, 0x18 mov rbx, rsp movq [rbx], offset .L1 mov [rbx+4], edi // Before the lcall, switch to a stack below 4GB. // This assumes that the data segment is below 4GB. mov rsp, offset stack+0xf0 lcall [rbx] // restore rsp to the original stack leave pop rbx ret .code32 .L1: call esi lret .code64 .globl check_mode // returns false for 32-bit mode; true for 64-bit mode check_mode: xor eax, eax // In 32-bit mode, this instruction is executed as // inc eax; test eax, eax test rax, rax setz al ret .data .align 16 stack: .space 0x100