1 | /* Copyright 1998 by the Massachusetts Institute of Technology. |
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2 | * |
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3 | * Permission to use, copy, modify, and distribute this |
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4 | * software and its documentation for any purpose and without |
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5 | * fee is hereby granted, provided that the above copyright |
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6 | * notice appear in all copies and that both that copyright |
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7 | * notice and this permission notice appear in supporting |
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8 | * documentation, and that the name of M.I.T. not be used in |
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9 | * advertising or publicity pertaining to distribution of the |
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10 | * software without specific, written prior permission. |
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11 | * M.I.T. makes no representations about the suitability of |
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12 | * this software for any purpose. It is provided "as is" |
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13 | * without express or implied warranty. |
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14 | */ |
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15 | |
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16 | /* |
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17 | * saferm |
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18 | * |
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19 | * Running as root, remove and optionally zero files in /tmp that |
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20 | * may be owned by anybody, avoiding any possible race conditions |
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21 | * resulting in security problems. |
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22 | */ |
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23 | |
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24 | static const char rcsid[] = "$Id: saferm.c,v 1.6 1999-09-21 20:09:11 danw Exp $"; |
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25 | |
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26 | /* See feature_test_macros(7) */ |
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27 | #define _FILE_OFFSET_BITS 64 |
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28 | |
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29 | #include <sys/types.h> |
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30 | #include <sys/stat.h> |
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31 | #include <fcntl.h> |
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32 | #include <string.h> |
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33 | #include <stdio.h> |
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34 | #include <unistd.h> |
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35 | #include <stdlib.h> |
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36 | #include <errno.h> |
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37 | |
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38 | char *program_name; |
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39 | |
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40 | #ifndef MIN |
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41 | #define MIN(a, b) ((a) < (b) ? (a) : (b)) |
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42 | #endif |
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43 | |
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44 | static int safe_open(char *filename, struct stat *file_stat); |
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45 | static int zero(int file, struct stat *file_stat); |
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46 | static int safe_to_unlink(char *name); |
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47 | static char *safe_cd(char *path); |
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48 | static int safe_rm(char *path, int zero_file); |
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49 | static int safe_rmdir(char *path, int quietflag); |
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50 | |
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51 | int main(int argc, char **argv) |
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52 | { |
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53 | int c, zeroflag = 0, dirflag = 0, quietflag = 0, errflag = 0; |
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54 | int dir, status = 0; |
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55 | |
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56 | /* Set up our program name. */ |
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57 | if (argv[0] != NULL) |
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58 | { |
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59 | program_name = strrchr(argv[0], '/'); |
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60 | if (program_name != NULL) |
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61 | program_name++; |
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62 | else |
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63 | program_name = argv[0]; |
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64 | } |
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65 | else |
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66 | program_name = "saferm"; |
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67 | |
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68 | while ((c = getopt(argc, argv, "dqz")) != EOF) |
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69 | { |
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70 | switch(c) |
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71 | { |
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72 | case 'd': |
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73 | dirflag = 1; |
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74 | break; |
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75 | case 'q': |
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76 | quietflag = 1; |
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77 | break; |
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78 | case 'z': |
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79 | zeroflag = 1; |
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80 | break; |
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81 | case '?': |
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82 | errflag = 1; |
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83 | break; |
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84 | } |
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85 | } |
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86 | |
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87 | if (errflag || optind == argc || (dirflag && zeroflag)) |
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88 | { |
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89 | fprintf(stderr, "usage: %s [-q] [-d|-z] filename [filename ...]\n", |
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90 | program_name); |
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91 | exit(1); |
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92 | } |
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93 | |
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94 | /* Remember our current directory in case we are being passed |
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95 | * relative paths. |
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96 | */ |
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97 | dir = open(".", O_RDONLY); |
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98 | if (dir == -1) |
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99 | { |
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100 | fprintf(stderr, "%s: error opening current directory: %s\n", |
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101 | program_name, strerror(errno)); |
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102 | exit(1); |
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103 | } |
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104 | |
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105 | while (optind < argc) |
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106 | { |
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107 | /* If we're removing a relative path, be sure to cd to where |
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108 | * we started. |
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109 | */ |
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110 | if (argv[optind][0] != '/') |
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111 | { |
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112 | if (fchdir(dir) == -1) |
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113 | { |
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114 | fprintf(stderr, "%s: error cding to initial directory: %s\n", |
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115 | program_name, strerror(errno)); |
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116 | exit(1); |
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117 | } |
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118 | } |
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119 | |
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120 | if (dirflag && safe_rmdir(argv[optind], quietflag) == -1) |
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121 | status = 1; |
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122 | else if (!dirflag && safe_rm(argv[optind], zeroflag) == -1) |
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123 | status = 1; |
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124 | |
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125 | optind++; |
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126 | } |
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127 | |
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128 | exit(status); |
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129 | } |
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130 | |
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131 | /* Safely open a file with intent to zero. */ |
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132 | static int safe_open(char *filename, struct stat *file_stat) |
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133 | { |
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134 | int file; |
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135 | struct stat name_stat; |
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136 | |
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137 | /* Try to open the file we think we want to zero. */ |
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138 | file = open(filename, O_WRONLY); |
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139 | if (file == -1) |
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140 | { |
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141 | fprintf(stderr, "%s: open(%s) for writing failed: %s\n", |
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142 | program_name, filename, strerror(errno)); |
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143 | return -1; |
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144 | } |
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145 | |
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146 | /* Stat the file we actually opened. */ |
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147 | if (fstat(file, file_stat) == -1) |
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148 | { |
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149 | fprintf(stderr, "%s: fstat(%s) failed: %s\n", |
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150 | program_name, filename, strerror(errno)); |
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151 | close(file); |
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152 | return -1; |
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153 | } |
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154 | |
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155 | /* Make sure it's a regular file. */ |
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156 | if (!S_ISREG(file_stat->st_mode)) |
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157 | { |
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158 | fprintf(stderr, "%s: %s is %s\n", |
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159 | program_name, filename, |
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160 | (S_ISDIR(file_stat->st_mode) |
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161 | ? "a directory" |
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162 | : "not a regular file")); |
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163 | close(file); |
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164 | return -1; |
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165 | } |
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166 | |
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167 | /* Stat the file we think we want to zero. */ |
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168 | if (lstat(filename, &name_stat) == -1) |
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169 | { |
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170 | fprintf(stderr, "%s: lstat(%s) failed: %s\n", |
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171 | program_name, filename, strerror(errno)); |
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172 | close(file); |
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173 | return -1; |
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174 | } |
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175 | |
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176 | /* We must be sure that we are not zeroing a hard link or symbolic |
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177 | * link to something like the password file. To do this, we must |
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178 | * verify that the link count on the file we zero is equal to 1, and |
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179 | * that we did not follow a symbolic link to reach that file. |
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180 | * |
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181 | * Simply lstat()ing the file before or after we open it is subject to |
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182 | * lose in a race condition, as there is no way to know that the file we |
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183 | * opened is the same file as the one we lstat()ed. Simply fstat()ing |
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184 | * the file we actually open can't tell us whether we followed a symbolic |
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185 | * link to get there, and also can't give us a reliable link count also |
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186 | * due to race conditions. |
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187 | * |
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188 | * But if we both lstat() and fstat() and compare the inode numbers from |
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189 | * the two cases, we can verify that the file we have open, at the time |
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190 | * of our lstat(), existed in a unique location identified by the pathname |
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191 | * we used to open it. |
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192 | * |
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193 | * So first, verify that our two stats got the same file. |
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194 | */ |
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195 | if (file_stat->st_dev != name_stat.st_dev |
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196 | || file_stat->st_ino != name_stat.st_ino) |
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197 | { |
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198 | fprintf(stderr, "%s: %s and file opened are not identical\n", |
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199 | program_name, filename); |
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200 | close(file); |
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201 | return -1; |
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202 | } |
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203 | |
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204 | /* Now verify that the link count is 1. Note that file_stat may not be |
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205 | * used here, otherwise we could be beaten by a race condition: |
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206 | * |
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207 | * 1. The attacker creates /tmp/passwd as a symlink to /etc/passwd. |
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208 | * 2. We open /tmp/passwd, which follows the symlink, and so we have |
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209 | * fstat()ed /etc/passwd. The link count as of this stat is 1. |
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210 | * 3. The attacker removes the /tmp/passwd symlink and replaces it |
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211 | * with a hard link to /etc/passwd. |
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212 | * 4. We lstat() /tmp/passwd. The link count now is 2. |
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213 | */ |
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214 | if (name_stat.st_nlink != 1) |
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215 | { |
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216 | fprintf(stderr, "%s: multiple links for file %s, not zeroing\n", |
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217 | program_name, filename); |
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218 | close(file); |
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219 | return -1; |
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220 | } |
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221 | |
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222 | /* Now we know it is safe to muck with the file we opened. */ |
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223 | return file; |
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224 | } |
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225 | |
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226 | /* Zero the file represented by the passed file descriptor. Assumes |
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227 | * that the file pointer is already at the beginning of the file, and |
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228 | * file_stat points to a stat of the file. |
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229 | */ |
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230 | static int zero(int file, struct stat *file_stat) |
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231 | { |
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232 | char zeroes[BUFSIZ]; |
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233 | off_t written = 0; |
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234 | ssize_t status; |
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235 | |
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236 | memset(zeroes, 0, BUFSIZ); |
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237 | |
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238 | while (written < file_stat->st_size) |
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239 | { |
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240 | status = write(file, zeroes, MIN(BUFSIZ, file_stat->st_size - written)); |
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241 | if (status == -1) |
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242 | { |
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243 | fprintf(stderr, "%s: write() failed: %s\n", |
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244 | program_name, strerror(errno)); |
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245 | return -1; |
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246 | } |
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247 | written += status; |
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248 | } |
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249 | |
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250 | fsync(file); |
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251 | return 0; |
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252 | } |
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253 | |
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254 | /* Return non-zero if it is safe to unlink NAME. */ |
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255 | static int safe_to_unlink(char *name) |
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256 | { |
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257 | struct stat st; |
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258 | |
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259 | if (lstat(name, &st) < 0) |
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260 | { |
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261 | fprintf(stderr, "%s: %s: %s\n", program_name, name, strerror(errno)); |
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262 | return 0; |
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263 | } |
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264 | |
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265 | if (S_ISDIR(st.st_mode)) |
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266 | { |
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267 | fprintf(stderr, "%s: %s is a directory\n", program_name, name); |
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268 | return 0; |
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269 | } |
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270 | |
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271 | return 1; |
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272 | } |
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273 | |
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274 | /* path is the full pathname to a file. We cd into the directory |
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275 | * containing the file, following no symbolic links. A pointer to just |
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276 | * the filename is returned on success (a substring of path), and NULL |
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277 | * is returned on failure. |
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278 | */ |
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279 | static char *safe_cd(char *path) |
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280 | { |
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281 | char *start_ptr, *end_ptr, *path_buf; |
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282 | struct stat above, below; |
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283 | |
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284 | if (path == NULL || *path == 0) |
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285 | return NULL; |
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286 | |
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287 | /* Make a copy of the path to be removed, since we're going to modify it. */ |
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288 | path_buf = malloc(strlen(path) + 1); |
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289 | if (path_buf == NULL) |
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290 | { |
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291 | fprintf(stderr, "%s: out of memory\n", program_name); |
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292 | return NULL; |
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293 | } |
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294 | strcpy(path_buf, path); |
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295 | |
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296 | /* Step through each path component and cd into it, verifying |
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297 | * that we wind up where we expect to. |
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298 | * |
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299 | * strchr()ing from start_ptr + 1 handles the absolute path case |
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300 | * as well as beginning to address the double-/ case. |
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301 | */ |
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302 | start_ptr = path_buf; |
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303 | while ((end_ptr = strchr(start_ptr + 1, '/'))) |
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304 | { |
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305 | /* Include any remaining extra /'s at the end of this component. */ |
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306 | while (*(end_ptr + 1) == '/') |
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307 | end_ptr++; |
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308 | |
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309 | *end_ptr = 0; |
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310 | |
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311 | if (lstat(start_ptr, &above) == -1) |
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312 | { |
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313 | fprintf(stderr, "%s: lstat of \"%s\" failed: %s\n", program_name, |
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314 | path_buf, strerror(errno)); |
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315 | free(path_buf); |
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316 | return NULL; |
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317 | } |
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318 | |
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319 | if (!S_ISDIR(above.st_mode)) |
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320 | { |
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321 | fprintf(stderr, "%s: final component of \"%s\" is not a directory\n", |
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322 | program_name, path_buf); |
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323 | free(path_buf); |
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324 | return NULL; |
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325 | } |
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326 | |
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327 | if (chdir(start_ptr) == -1) |
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328 | { |
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329 | fprintf(stderr, "%s: chdir to \"%s\" failed: %s\n", program_name, |
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330 | path_buf, strerror(errno)); |
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331 | free(path_buf); |
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332 | return NULL; |
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333 | } |
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334 | |
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335 | if (stat(".", &below) == -1) |
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336 | { |
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337 | fprintf(stderr, "%s: stat of \"%s\" failed: %s\n", program_name, |
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338 | path_buf, strerror(errno)); |
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339 | free(path_buf); |
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340 | return NULL; |
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341 | } |
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342 | |
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343 | if (above.st_dev != below.st_dev || above.st_ino != below.st_ino) |
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344 | { |
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345 | fprintf(stderr, "%s: final component of \"%s\" changed during " |
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346 | "traversal\n", program_name, path_buf); |
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347 | free(path_buf); |
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348 | return NULL; |
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349 | } |
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350 | |
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351 | /* Advance to next component. */ |
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352 | start_ptr = end_ptr + 1; |
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353 | |
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354 | /* Restore the / we nulled for diagnostics. */ |
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355 | *end_ptr = '/'; |
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356 | } |
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357 | |
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358 | free(path_buf); |
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359 | return (start_ptr - path_buf) + path; |
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360 | } |
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361 | |
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362 | /* Safely remove and optionally zero a file. -1 is returned for any |
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363 | * level of failure, such as zeroing failed but unlinking succeeded. |
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364 | */ |
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365 | static int safe_rm(char *path, int zero_file) |
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366 | { |
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367 | int file, status = 0; |
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368 | struct stat file_stat; |
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369 | char *name; |
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370 | |
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371 | name = safe_cd(path); |
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372 | if (name == NULL) |
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373 | return -1; |
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374 | |
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375 | /* Do the unlink before the actual zeroing so that the appearance |
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376 | * is atomic. |
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377 | */ |
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378 | |
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379 | if (zero_file) |
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380 | { |
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381 | file = safe_open(name, &file_stat); |
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382 | if (file == -1) |
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383 | return -1; |
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384 | } |
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385 | |
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386 | if (safe_to_unlink(name) && unlink(name) == -1) |
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387 | { |
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388 | fprintf(stderr, "%s: error unlinking %s: %s\n", program_name, |
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389 | path, strerror(errno)); |
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390 | status = -1; |
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391 | } |
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392 | |
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393 | if (zero_file && file != -1) |
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394 | { |
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395 | if (zero(file, &file_stat) == -1) |
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396 | status = -1; |
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397 | close(file); |
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398 | } |
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399 | |
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400 | return status; |
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401 | } |
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402 | |
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403 | /* Safely remove a directory. -1 is returned for any level of failure. */ |
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404 | static int safe_rmdir(char *path, int quietflag) |
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405 | { |
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406 | char *name; |
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407 | |
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408 | name = safe_cd(path); |
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409 | if (name == NULL) |
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410 | return -1; |
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411 | |
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412 | if (rmdir(name) == -1) |
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413 | { |
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414 | /* System V systems return EEXIST when a directory isn't empty |
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415 | * but hack the rmdir command to display a better error message. |
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416 | * Well, we can do that too. |
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417 | */ |
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418 | if (errno == EEXIST) |
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419 | errno = ENOTEMPTY; |
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420 | |
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421 | if (!quietflag || errno != ENOTEMPTY) |
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422 | { |
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423 | fprintf(stderr, "%s: error removing directory %s: %s\n", |
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424 | program_name, path, strerror(errno)); |
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425 | } |
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426 | return -1; |
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427 | } |
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428 | |
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429 | return 0; |
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430 | } |
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