buffer.c

/*
 * Dropbear SSH
 *
 * Copyright (c) 2002,2003 Matt Johnston
 * All rights reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

/* Buffer handling routines, designed to avoid overflows/using invalid data */

#include "includes.h"
#include "dbutil.h"
#include "buffer.h"

/* Prevent integer overflows when incrementing buffer position/length.
 * Calling functions should check arguments first, but this provides a
 * backstop */
#define BUF_MAX_INCR 1000000000
#define BUF_MAX_SIZE 1000000000

/* avoid excessively large numbers, > ~8192 bits */
#define BUF_MAX_MPINT (8240 / 8)

/* Create (malloc) a new buffer of size */
buffer* buf_new(unsigned int size) {
  buffer* buf;

  if (size > BUF_MAX_SIZE) {
    dropbear_exit("buf->size too big");
  }

  buf = (buffer*)m_malloc(sizeof(buffer) + size);

  if (size > 0) {
    buf->data = (unsigned char*)buf + sizeof(buffer);
  } else {
    buf->data = NULL;
  }

  buf->size = size;

  return buf;
}

/* free the buffer's data and the buffer itself */
void buf_free(buffer* buf) { m_free(buf); }

/* overwrite the contents of the buffer to clear it */
void buf_burn(buffer* buf) { m_burn(buf->data, buf->size); }

/* resize a buffer, pos and len will be repositioned if required when
 * downsizing */
buffer* buf_resize(buffer* buf, unsigned int newsize) {
  if (newsize > BUF_MAX_SIZE) {
    dropbear_exit("buf->size too big");
  }

  buf = m_realloc(buf, sizeof(buffer) + newsize);
  buf->data = (unsigned char*)buf + sizeof(buffer);
  buf->size = newsize;
  buf->len = MIN(newsize, buf->len);
  buf->pos = MIN(newsize, buf->pos);
  return buf;
}

/* Create a copy of buf, allocating required memory etc. */
/* The new buffer is sized the same as the length of the source buffer. */
buffer* buf_newcopy(buffer* buf) {
  buffer* ret;

  ret = buf_new(buf->len);
  ret->len = buf->len;
  if (buf->len > 0) {
    memcpy(ret->data, buf->data, buf->len);
  }
  return ret;
}

/* Set the length of the buffer */
void buf_setlen(buffer* buf, unsigned int len) {
  if (len > buf->size) {
    dropbear_exit("Bad buf_setlen");
  }
  buf->len = len;
}

/* Increment the length of the buffer */
void buf_incrlen(buffer* buf, unsigned int incr) {
  if (incr > BUF_MAX_INCR || buf->len + incr > buf->size) {
    dropbear_exit("Bad buf_incrlen");
  }
  buf->len += incr;
}
/* Set the position of the buffer */
void buf_setpos(buffer* buf, unsigned int pos) {
  if (pos > buf->len) {
    dropbear_exit("Bad buf_setpos");
  }
  buf->pos = pos;
}

/* increment the position by incr, increasing the buffer length if required */
void buf_incrwritepos(buffer* buf, unsigned int incr) {
  if (incr > BUF_MAX_INCR || buf->pos + incr > buf->size) {
    dropbear_exit("Bad buf_incrwritepos");
  }
  buf->pos += incr;
  if (buf->pos > buf->len) {
    buf->len = buf->pos;
  }
}

/* increment the position by incr, negative values are allowed, to
 * decrement the pos*/
void buf_incrpos(buffer* buf, int incr) {
  if (incr > BUF_MAX_INCR || incr < -BUF_MAX_INCR ||
      (unsigned int)((int)buf->pos + incr) > buf->len ||
      ((int)buf->pos + incr) < 0) {
    dropbear_exit("Bad buf_incrpos");
  }
  buf->pos += incr;
}

/* Get a byte from the buffer and increment the pos */
unsigned char buf_getbyte(buffer* buf) {
  /* This check is really just ==, but the >= allows us to check for the
   * bad case of pos > len, which should _never_ happen. */
  if (buf->pos >= buf->len) {
    dropbear_exit("Bad buf_getbyte");
  }
  return buf->data[buf->pos++];
}

/* Get a bool from the buffer and increment the pos */
unsigned char buf_getbool(buffer* buf) {
  unsigned char b;
  b = buf_getbyte(buf);
  if (b != 0) b = 1;
  return b;
}

/* put a byte, incrementing the length if required */
void buf_putbyte(buffer* buf, unsigned char val) {
  if (buf->pos >= buf->len) {
    buf_incrlen(buf, 1);
  }
  buf->data[buf->pos] = val;
  buf->pos++;
}

/* returns an in-place pointer to the buffer, checking that
 * the next len bytes from that position can be used */
unsigned char* buf_getptr(buffer* buf, unsigned int len) {
  if (len > BUF_MAX_INCR || buf->pos + len > buf->len) {
    dropbear_exit("Bad buf_getptr");
  }
  return &buf->data[buf->pos];
}

/* like buf_getptr, but checks against total size, not used length.
 * This allows writing past the used length, but not past the size */
unsigned char* buf_getwriteptr(buffer* buf, unsigned int len) {
  if (len > BUF_MAX_INCR || buf->pos + len > buf->size) {
    dropbear_exit("Bad buf_getwriteptr");
  }
  return &buf->data[buf->pos];
}

/* Return a null-terminated string, it is malloced, so must be free()ed
 * Note that the string isn't checked for null bytes, hence the retlen
 * may be longer than what is returned by strlen */
char* buf_getstring(buffer* buf, unsigned int* retlen) {
  unsigned int len;
  char* ret;
  len = buf_getint(buf);
  if (len > MAX_STRING_LEN) {
    dropbear_exit("String too long");
  }

  if (retlen != NULL) {
    *retlen = len;
  }
  ret = m_malloc(len + 1);
  memcpy(ret, buf_getptr(buf, len), len);
  buf_incrpos(buf, len);
  ret[len] = '\0';

  return ret;
}

/* Return a string as a newly allocated buffer */
buffer* buf_getstringbuf(buffer* buf) {
  buffer* ret = NULL;
  unsigned int len = buf_getint(buf);
  if (len > MAX_STRING_LEN) {
    dropbear_exit("String too long");
  }
  ret = buf_new(len);
  memcpy(buf_getwriteptr(ret, len), buf_getptr(buf, len), len);
  buf_incrpos(buf, len);
  buf_incrlen(ret, len);
  return ret;
}

/* Just increment the buffer position the same as if we'd used buf_getstring,
 * but don't bother copying/malloc()ing for it */
void buf_eatstring(buffer* buf) { buf_incrpos(buf, buf_getint(buf)); }

/* Get an uint32 from the buffer and increment the pos */
unsigned int buf_getint(buffer* buf) {
  unsigned int ret;

  LOAD32H(ret, buf_getptr(buf, 4));
  buf_incrpos(buf, 4);
  return ret;
}

/* put a 32bit uint into the buffer, incr bufferlen & pos if required */
void buf_putint(buffer* buf, int unsigned val) {
  STORE32H(val, buf_getwriteptr(buf, 4));
  buf_incrwritepos(buf, 4);
}

/* put a SSH style string into the buffer, increasing buffer len if required */
void buf_putstring(buffer* buf, const char* str, unsigned int len) {
  buf_putint(buf, len);
  buf_putbytes(buf, (const unsigned char*)str, len);
}

/* puts an entire buffer as a SSH string. ignore pos of buf_str. */
void buf_putbufstring(buffer* buf, const buffer* buf_str) {
  buf_putstring(buf, (const char*)buf_str->data, buf_str->len);
}

/* put the set of len bytes into the buffer, incrementing the pos, increasing
 * len if required */
void buf_putbytes(buffer* buf, const unsigned char* bytes, unsigned int len) {
  memcpy(buf_getwriteptr(buf, len), bytes, len);
  buf_incrwritepos(buf, len);
}

/* for our purposes we only need positive (or 0) numbers, so will
 * fail if we get negative numbers */
void buf_putmpint(buffer* buf, mp_int* mp) {
  unsigned int len, pad = 0;
  TRACE2(("enter buf_putmpint"))

  dropbear_assert(mp != NULL);

  if (SIGN(mp) == MP_NEG) {
    dropbear_exit("negative bignum");
  }

  /* zero check */
  if (USED(mp) == 1 && DIGIT(mp, 0) == 0) {
    len = 0;
  } else {
    /* SSH spec requires padding for mpints with the MSB set, this code
     * implements it */
    len = mp_count_bits(mp);
    /* if the top bit of MSB is set, we need to pad */
    pad = (len % 8 == 0) ? 1 : 0;
    len = len / 8 + 1; /* don't worry about rounding, we need it for
                                              padding anyway when len%8 == 0 */
  }

  /* store the length */
  buf_putint(buf, len);

  /* store the actual value */
  if (len > 0) {
    if (pad) {
      buf_putbyte(buf, 0x00);
    }
    if (mp_to_unsigned_bin(mp, buf_getwriteptr(buf, len - pad)) != MP_OKAY) {
      dropbear_exit("mpint error");
    }
    buf_incrwritepos(buf, len - pad);
  }

  TRACE2(("leave buf_putmpint"))
}

/* Retrieve an mp_int from the buffer.
 * Will fail for -ve since they shouldn't be required here.
 * Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
int buf_getmpint(buffer* buf, mp_int* mp) {
  unsigned int len;
  len = buf_getint(buf);

  if (len == 0) {
    mp_zero(mp);
    return DROPBEAR_SUCCESS;
  }

  if (len > BUF_MAX_MPINT) {
    return DROPBEAR_FAILURE;
  }

  /* check for negative */
  if (*buf_getptr(buf, 1) & (1 << (CHAR_BIT - 1))) {
    return DROPBEAR_FAILURE;
  }

  if (mp_read_unsigned_bin(mp, buf_getptr(buf, len), len) != MP_OKAY) {
    return DROPBEAR_FAILURE;
  }

  buf_incrpos(buf, len);
  return DROPBEAR_SUCCESS;
}