darkhttpd/devel/fuzz_llvm_make_safe_uri.c

/* Wrapper around make_safe_url() for fuzzing with LLVM.
 * Aborts if the output is deemed safe but contains /../ or /./
 */
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define main darkhttpd_main
#include "../darkhttpd.c"
#undef main

/* This is the code for the older / simpler make_safe_uri.
 * The new one comes from ../darkhttpd.c included above.
 */

/* Consolidate slashes in-place by shifting parts of the string over repeated
 * slashes.
 */
static void consolidate_slashes(char *s) {
  size_t left = 0, right = 0;
  int saw_slash = 0;

  assert(s != NULL);
  while (s[right] != '\0') {
    if (saw_slash) {
      if (s[right] == '/')
        right++;
      else {
        saw_slash = 0;
        s[left++] = s[right++];
      }
    } else {
      if (s[right] == '/') saw_slash++;
      s[left++] = s[right++];
    }
  }
  s[left] = '\0';
}

/* Resolve /./ and /../ in a URL, in-place.  Also strip out query params.
 * Returns NULL if the URL is invalid/unsafe, or the original buffer if
 * successful.
 */
static char *make_safe_url_old(char *url) {
  struct {
    char *start;
    size_t len;
  } * chunks;
  unsigned int num_slashes, num_chunks;
  size_t urllen, i, j, pos;
  int ends_in_slash;

  /* query params are now stripped in process_get, not in make_safe_url */
#if 0
  /* strip query params */
  for (pos = 0; url[pos] != '\0'; pos++) {
    if (url[pos] == '?') {
      url[pos] = '\0';
      break;
    }
  }
#endif

  if (url[0] != '/') return NULL;

  consolidate_slashes(url);
  urllen = strlen(url);
  if (urllen > 0)
    ends_in_slash = (url[urllen - 1] == '/');
  else
    ends_in_slash = 1;

  /* count the slashes */
  for (i = 0, num_slashes = 0; i < urllen; i++)
    if (url[i] == '/') num_slashes++;

  /* make an array for the URL elements */
  assert(num_slashes > 0);
  chunks = malloc(sizeof(*chunks) * num_slashes);

  /* split by slashes and build chunks array */
  num_chunks = 0;
  for (i = 1; i < urllen;) {
    /* look for the next slash */
    for (j = i; j < urllen && url[j] != '/'; j++)
      ;

    /* process url[i,j) */
    if ((j == i + 1) && (url[i] == '.'))
      /* "." */;
    else if ((j == i + 2) && (url[i] == '.') && (url[i + 1] == '.')) {
      /* ".." */
      if (num_chunks == 0) {
        /* unsafe string so free chunks */
        free(chunks);
        return (NULL);
      } else
        num_chunks--;
    } else {
      chunks[num_chunks].start = url + i;
      chunks[num_chunks].len = j - i;
      num_chunks++;
    }

    i = j + 1; /* url[j] is a slash - move along one */
  }

  /* reassemble in-place */
  pos = 0;
  for (i = 0; i < num_chunks; i++) {
    assert(pos <= urllen);
    url[pos++] = '/';

    assert(pos + chunks[i].len <= urllen);
    assert(url + pos <= chunks[i].start);

    if (url + pos < chunks[i].start)
      memmove(url + pos, chunks[i].start, chunks[i].len);
    pos += chunks[i].len;
  }
  free(chunks);

  if ((num_chunks == 0) || ends_in_slash) url[pos++] = '/';
  assert(pos <= urllen);
  url[pos] = '\0';
  return url;
}

char* sdup(const uint8_t *data, size_t size) {
  char *buf = malloc(size + 1);
  memcpy(buf, data, size);
  buf[size] = 0;
  return buf;
}

void check(char* safe) {
  if (safe == NULL) return;
  if (strstr(safe, "/../") != NULL) __builtin_trap();
  if (strstr(safe, "/./") != NULL) __builtin_trap();
}

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
  char *buf1 = sdup(data, size);
  char *buf2 = sdup(data, size);
  // Enable this to make sure the fuzzer is working.
#if 0
  if (size > 2 && buf1[2] == 'x') __builtin_trap();
#endif
  char *safe1 = make_safe_url_old(buf1);
  char *safe2 = make_safe_url(buf2);
  check(safe1);
  check(safe2);
  if (safe1 == NULL) {
    if (safe2 != NULL) __builtin_trap();
  } else {
    if (strcmp(safe1, safe2) != 0) {
      printf("ERROR: Mismatch: old [%s] new [%s]\n", safe1, safe2);
      __builtin_trap();
    }
  }
  // Don't leak memory.
  free(buf1);
  free(buf2);
  return 0;
}

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