gs_gdt.c

/*

Sample code to setup a GDT, and use segments.

Copyright(c) 2016 Chris Eagle

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
version 2 as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

*/

#include <unicorn/unicorn.h>
#include <string.h>
#include <assert.h>
#include <stdlib.h>

#pragma pack(push, 1)
struct SegmentDescriptor {
   union {
      struct {
         unsigned short limit0;
         unsigned short base0;
         unsigned char base1;
         unsigned char type:4;
         unsigned char system:1;      /* S flag */
         unsigned char dpl:2;
         unsigned char present:1;     /* P flag */
         unsigned char limit1:4;
         unsigned char avail:1;
         unsigned char is_64_code:1;  /* L flag */
         unsigned char db:1;          /* DB flag */
         unsigned char granularity:1; /* G flag */
         unsigned char base2;
      };
      uint64_t desc;
   };
};
#pragma pack(pop)

#define SEGBASE(d) ((uint32_t)((((d).desc >> 16) & 0xffffff) | (((d).desc >> 32) & 0xff000000)))
#define SEGLIMIT(d) ((d).limit0 | (((unsigned int)(d).limit1) << 16))

/**
 * Assert that err matches expect
 */
#define uc_assert_err(expect, err)                                  \
do {                                                                \
    uc_err __err = err;                                             \
    if (__err != expect) {                                          \
        fprintf(stderr, "%s", uc_strerror(__err));                  \
        exit(1);                                                    \
    }                                                               \
} while (0)

/**
 * Assert that err is UC_ERR_OK
 */
#define uc_assert_success(err)  uc_assert_err(UC_ERR_OK, err)

/**
 * Assert that err is anything but UC_ERR_OK
 *
 * Note: Better to use uc_assert_err(<specific error>, err),
 * as this serves to document which errors a function will return
 * in various scenarios.
 */
#define uc_assert_fail(err)                                         \
do {                                                                \
    uc_err __err = err;                                             \
    if (__err == UC_ERR_OK) {                                       \
        fprintf(stderr, "%s", uc_strerror(__err));                  \
        exit(1);                                                    \
    }                                                               \
} while (0)

#define OK(x)   uc_assert_success(x)

/******************************************************************************/

static void hook_mem(uc_engine *uc, uc_mem_type type, uint64_t address, int size, int64_t value, void *user_data)
{
  uint64_t esp = 0;
  uc_reg_read(uc, UC_X86_REG_ESP, &esp);
  printf("mem access: address: 0x%"PRIx64 " esp 0x%"PRIx64 "\n", address, esp);
}

static void hook_code(uc_engine *uc, uint64_t address, uint32_t size, void *user_data)
{
    printf("Executing at 0x%"PRIx64 ", ilen = 0x%x\n", address, size);
}

//VERY basic descriptor init function, sets many fields to user space sane defaults
static void init_descriptor(struct SegmentDescriptor *desc, uint32_t base, uint32_t limit, uint8_t is_code)
{
    desc->desc = 0;  //clear the descriptor
    desc->base0 = base & 0xffff;
    desc->base1 = (base >> 16) & 0xff;
    desc->base2 = base >> 24;
    if (limit > 0xfffff) {
        //need Giant granularity
        limit >>= 12;
        desc->granularity = 1;
    }
    desc->limit0 = limit & 0xffff;
    desc->limit1 = limit >> 16;

    //some sane defaults
    desc->dpl = 3;
    desc->present = 1;
    desc->db = 1;   //32 bit
    desc->type = is_code ? 0xb : 3;
    desc->system = 1;  //code or data
}

static void gdt_demo()
{
    uc_engine *uc;
    uc_hook hook1, hook2;
    uc_err err;

    uc_x86_mmr gdtr;


    /*
       bits 32

       pop esi
     */

    const uint8_t code[] = "\x5e";
    const uint64_t code_address = 0x1000000;
    const uint64_t stack_address = 0x120000;
    const uint64_t gdt_address = 0xc0000000;

    struct SegmentDescriptor *gdt = (struct SegmentDescriptor*)calloc(31, sizeof(struct SegmentDescriptor));

    int r_esp = (int)stack_address + 0x100;     // initial esp

    gdtr.base = gdt_address;
    gdtr.limit = 31 * sizeof(struct SegmentDescriptor) - 1;

    init_descriptor(&gdt[1], 0, 0xfffff000, 1);  //gs

    // Initialize emulator in X86-32bit mode
    err = uc_open(UC_ARCH_X86, UC_MODE_32, &uc);
    uc_assert_success(err);

    uc_hook_add(uc, &hook1, UC_HOOK_CODE, hook_code, NULL, code_address, code_address + sizeof(code) - 1);

    err = uc_hook_add(uc, &hook2, UC_HOOK_MEM_UNMAPPED, hook_mem, NULL, (uint64_t)1, (uint64_t)0);
    uc_assert_success(err);

    // map 1 page of code for this emulation
    err = uc_mem_map(uc, code_address, 0x1000, UC_PROT_ALL);
    uc_assert_success(err);

    // map 1 page of stack for this emulation
    err = uc_mem_map(uc, stack_address, 0x1000, UC_PROT_READ | UC_PROT_WRITE);
    uc_assert_success(err);

    // map 64k for a GDT
    err = uc_mem_map(uc, gdt_address, 0x10000, UC_PROT_WRITE | UC_PROT_READ);
    uc_assert_success(err);

    //set up a GDT BEFORE you manipulate any segment registers
    err = uc_reg_write(uc, UC_X86_REG_GDTR, &gdtr);
    uc_assert_success(err);

    // write gdt to be emulated to memory
    err = uc_mem_write(uc, gdt_address, gdt, 31 * sizeof(struct SegmentDescriptor));
    uc_assert_success(err);

     // write machine code to be emulated to memory
    err = uc_mem_write(uc, code_address, code, sizeof(code)-1);
    uc_assert_success(err);

    // initialize machine registers
    err = uc_reg_write(uc, UC_X86_REG_ESP, &r_esp);
    uc_assert_success(err);


    int r_gs = 1<<3;
    err = uc_reg_write(uc, UC_X86_REG_GS, &r_gs);
    uc_assert_success(err);

    // emulate machine code in infinite time
    err = uc_emu_start(uc, code_address, code_address+sizeof(code)-1, 0, 0);
    uc_assert_success(err);

    uc_close(uc);
}

/******************************************************************************/

int main(int argc, char **argv)
{
    gdt_demo();

    fprintf(stderr, "success\n");

    return 0;
}

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