release/src/router/vsftpd/secbuf.c

   1 /*
   2  * Part of Very Secure FTPd
   3  * Licence: GPL v2
   4  * Author: Chris Evans
   5  * secbuf.c
   6  *
   7  * Here are some routines providing the (possibly silly) concept of a secure
   8  * buffer. A secure buffer may not be overflowed. A single byte overflow
   9  * will cause the program to safely terminate.
  10  */
  11
  12 #include "secbuf.h"
  13 #include "utility.h"
  14 #include "sysutil.h"
  15 #include "sysdeputil.h"
  16
  17 void
  18 vsf_secbuf_alloc(char** p_ptr, unsigned int size)
  19 {
  20   unsigned int page_offset;
  21   unsigned int round_up;
  22   char* p_mmap;
  23   char* p_no_access_page;
  24   unsigned int page_size = vsf_sysutil_getpagesize();
  25
  26   /* Free any previous buffer */
  27   vsf_secbuf_free(p_ptr);
  28   /* Round up to next page size */
  29   page_offset = size % page_size;
  30   if (page_offset)
  31   {
  32     unsigned int num_pages = size / page_size;
  33     num_pages++;
  34     round_up = num_pages * page_size;
  35   }
  36   else
  37   {
  38     /* Allocation is on a page-size boundary */
  39     round_up = size;
  40   }
  41   /* Add on another two pages to make inaccessible */
  42   round_up += page_size * 2;
  43
  44   p_mmap = vsf_sysutil_map_anon_pages(round_up);
  45   /* Map the first and last page inaccessible */
  46   p_no_access_page = p_mmap + round_up - page_size;
  47   vsf_sysutil_memprotect(p_no_access_page, page_size, kVSFSysUtilMapProtNone);
  48   /* Before we make the "before" page inaccessible, store the size in it.
  49    * A little hack so that we don't need to explicitly be passed the size
  50    * when freeing an existing secure buffer
  51    */
  52   *((unsigned int*)p_mmap) = round_up;
  53   p_no_access_page = p_mmap;
  54   vsf_sysutil_memprotect(p_no_access_page, page_size, kVSFSysUtilMapProtNone);
  55
  56   p_mmap += page_size;
  57   if (page_offset)
  58   {
  59     p_mmap += (page_size - page_offset);
  60   }
  61   *p_ptr = p_mmap;
  62 }
  63
  64 void
  65 vsf_secbuf_free(char** p_ptr)
  66 {
  67   unsigned int map_size;
  68   unsigned long page_offset;
  69   char* p_mmap = *p_ptr;
  70   unsigned int page_size = vsf_sysutil_getpagesize();
  71   if (p_mmap == 0)
  72   {
  73     return;
  74   }
  75   /* Calculate the actual start of the mmap region */
  76   page_offset = (unsigned long) p_mmap % page_size;
  77   if (page_offset)
  78   {
  79     p_mmap -= page_offset;
  80   }
  81   p_mmap -= page_size;
  82   /* First make the first page readable so we can get the size */
  83   vsf_sysutil_memprotect(p_mmap, page_size, kVSFSysUtilMapProtReadOnly);
  84   /* Extract the mapping size */
  85   map_size = *((unsigned int*)p_mmap);
  86   /* Lose the mapping */
  87   vsf_sysutil_memunmap(p_mmap, map_size);
  88 }
  89