getapi.c

/**
  Copyright © 2017 Odzhan. All Rights Reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:

  1. Redistributions of source code must retain the above copyright
  notice, this list of conditions and the following disclaimer.

  2. Redistributions in binary form must reproduce the above copyright
  notice, this list of conditions and the following disclaimer in the
  documentation and/or other materials provided with the distribution.

  3. The name of the author may not be used to endorse or promote products
  derived from this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY AUTHORS "AS IS" AND ANY EXPRESS OR
  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
  INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  POSSIBILITY OF SUCH DAMAGE. */

#include "peb.h"
  
LPVOID get_api(DWORD);
LPVOID get_apix(DWORD);

// converts string to lowercase
uint32_t crc32c(const char *s) {
    int      i;
    uint32_t crc=0;
    
    while (*s) {
      crc ^= (uint8_t)(*s++ | 0x20);
      
      for (i=0; i<8; i++) {
        crc = (crc >> 1) ^ (0x82F63B78 * (crc & 1));
      }
    }
    return crc;
}

#ifndef ASM
LPVOID search_exp(LPVOID base, DWORD hash) {
    PIMAGE_DOS_HEADER       dos;
    PIMAGE_NT_HEADERS       nt;
    DWORD                   cnt, rva, dll_h;
    PIMAGE_DATA_DIRECTORY   dir;
    PIMAGE_EXPORT_DIRECTORY exp;
    PDWORD                  adr;
    PDWORD                  sym;
    PWORD                   ord;
    PCHAR                   api, dll;
    LPVOID                  api_adr=NULL;
    
    dos = (PIMAGE_DOS_HEADER)base;
    nt  = RVA2VA(PIMAGE_NT_HEADERS, base, dos->e_lfanew);
    dir = (PIMAGE_DATA_DIRECTORY)nt->OptionalHeader.DataDirectory;
    rva = dir[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress;
    
    // if no export table, return NULL
    if (rva==0) return NULL;
    
    exp = (PIMAGE_EXPORT_DIRECTORY) RVA2VA(ULONG_PTR, base, rva);
    cnt = exp->NumberOfNames;
    
    // if no api names, return NULL
    if (cnt==0) return NULL;
    
    adr = RVA2VA(PDWORD,base, exp->AddressOfFunctions);
    sym = RVA2VA(PDWORD,base, exp->AddressOfNames);
    ord = RVA2VA(PWORD, base, exp->AddressOfNameOrdinals);
    dll = RVA2VA(PCHAR, base, exp->Name);
    
    // calculate hash of DLL string
    dll_h = crc32c(dll);
    
    do {
      // calculate hash of api string
      api = RVA2VA(PCHAR, base, sym[cnt-1]);
      // add to DLL hash and compare
      if (crc32c(api) + dll_h == hash) {
        // return address of function
        api_adr = RVA2VA(LPVOID, base, adr[ord[cnt-1]]);
        return api_adr;
      }
    } while (--cnt && api_adr==0);
    return api_adr;
}

LPVOID search_imp(LPVOID base, DWORD hash) {
    DWORD                    dll_h, i, rva;
    PIMAGE_IMPORT_DESCRIPTOR imp;
    PIMAGE_THUNK_DATA        oft, ft;
    PIMAGE_IMPORT_BY_NAME    ibn;
    PIMAGE_DOS_HEADER        dos;
    PIMAGE_NT_HEADERS        nt;
    PIMAGE_DATA_DIRECTORY    dir;
    PCHAR                    dll;
    LPVOID                   api_adr=NULL;
    
    dos = (PIMAGE_DOS_HEADER)base;
    nt  = RVA2VA(PIMAGE_NT_HEADERS, base, dos->e_lfanew);
    dir = (PIMAGE_DATA_DIRECTORY)nt->OptionalHeader.DataDirectory;
    rva = dir[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress;
    
    // if no import table, return
    if (rva==0) return NULL;

    imp = RVA2VA(PIMAGE_IMPORT_DESCRIPTOR, base, rva);
    
    for (i=0; api_adr==NULL; i++) {
      // no more DLL to process?
      if (imp[i].Name == 0) return NULL;
      
      // calculate crc32c hash of DLL string
      dll   = RVA2VA(PCHAR, base, imp[i].Name);
      dll_h = crc32c(dll); 
      
      // obtain address of API names
      rva   = imp[i].OriginalFirstThunk;
      oft   = RVA2VA(PIMAGE_THUNK_DATA, base, rva);
      
      // obtain address of API addresses
      rva   = imp[i].FirstThunk;
      ft    = RVA2VA(PIMAGE_THUNK_DATA, base, rva);
          
      for (;; oft++, ft++) {
        if (oft->u1.Ordinal == 0) break;
        
        // skip import by ordinal
        if (IMAGE_SNAP_BY_ORDINAL(oft->u1.Ordinal)) continue;
        
        // obtain address of API string
        rva = oft->u1.AddressOfData;
        ibn = RVA2VA(PIMAGE_IMPORT_BY_NAME, base, rva);
        
        if ((crc32c((char*)ibn->Name) + dll_h) == hash) {
          api_adr = (LPVOID)ft->u1.Function;
          break;
        }
      }
    }
    return api_adr;
}
 
/**F*********************************************
 *
 * Obtain address of API from PEB based on hash
 *
 ************************************************/
LPVOID get_api (DWORD dwHash) {
    PPEB                  peb;
    PPEB_LDR_DATA         ldr;
    PLDR_DATA_TABLE_ENTRY dte;
    LPVOID                api_adr=NULL;
    
    peb  = NtCurrentTeb()->ProcessEnvironmentBlock;
    ldr = (PPEB_LDR_DATA)peb->Ldr;
    
    // for each DLL loaded
    for (dte=(PLDR_DATA_TABLE_ENTRY)ldr->InLoadOrderModuleList.Flink;
         dte->DllBase != NULL && api_adr == NULL; 
         dte=(PLDR_DATA_TABLE_ENTRY)dte->InLoadOrderLinks.Flink)
    {
  #ifdef IMPORT    
      api_adr=search_imp(dte->DllBase, dwHash);
  #else    
      api_adr=search_exp(dte->DllBase, dwHash);
  #endif  
    }
    return api_adr;
}
#endif

#ifdef TEST
int main(int argc, char *argv[])
{
  DWORD  h, dll_h, api_h;
  LPVOID p=NULL;
  
  if (argc!=3) {
    printf("\nusage: getapi <DLL> <API>\n");
    return 0;
  }
  dll_h = crc32c(argv[1]);
  api_h = crc32c(argv[2]);
  
  h = dll_h + api_h;
  
  p = get_apix(h);
  // if not found
  if (p==NULL) {
    // load the module into memory
    if (LoadLibrary(argv[1])==NULL) {
      printf ("\nUnable to load DLL module \"%s\"", argv[1]);
      return 0;
    }
    // then try again
    p = get_apix(h);
  }
  if (p==NULL) {
    printf ("\nUnable to locate API address \"%s\"", argv[2]);
    return 0;
  }
  printf("\n%08lX = crc-32c(\"%s\")"
         "\n%08lX = crc-32c(\"%s\")"
         "\n%08lX = hash to find"
         "\n%p = Address of \"%s\"\n", 
      dll_h, argv[1], 
      api_h, argv[2],
      h,
      p, argv[2]);
      
  return 0;    
}
#endif