1 /* 2 * safe_finger - finger client wrapper that protects against nasty stuff 3 * from finger servers. Use this program for automatic reverse finger 4 * probes, not the raw finger command. 5 * 6 * Build with: cc -o safe_finger safe_finger.c 7 * 8 * The problem: some programs may react to stuff in the first column. Other 9 * programs may get upset by thrash anywhere on a line. File systems may 10 * fill up as the finger server keeps sending data. Text editors may bomb 11 * out on extremely long lines. The finger server may take forever because 12 * it is somehow wedged. The code below takes care of all this badness. 13 * 14 * Author: Wietse Venema, Eindhoven University of Technology, The Netherlands. 15 */ 16 17 #ifndef lint 18 static char sccsid[] = "@(#) safe_finger.c 1.4 94/12/28 17:42:41"; 19 #endif 20 21 /* System libraries */ 22 23 #include <sys/types.h> 24 #include <sys/stat.h> 25 #include <signal.h> 26 #include <stdio.h> 27 #include <ctype.h> 28 #include <pwd.h> 29 30 extern void exit(); 31 32 /* Local stuff */ 33 34 char path[] = "PATH=/bin:/usr/bin:/usr/ucb:/usr/bsd:/etc:/usr/etc:/usr/sbin"; 35 36 #define TIME_LIMIT 60 /* Do not keep listinging forever */ 37 #define INPUT_LENGTH 100000 /* Do not keep listinging forever */ 38 #define LINE_LENGTH 128 /* Editors can choke on long lines */ 39 #define FINGER_PROGRAM "finger" /* Most, if not all, UNIX systems */ 40 #define UNPRIV_NAME "nobody" /* Preferred privilege level */ 41 #define UNPRIV_UGID 32767 /* Default uid and gid */ 42 43 int finger_pid; 44 45 void cleanup(sig) 46 int sig; 47 { 48 kill(finger_pid, SIGKILL); 49 exit(0); 50 } 51 52 main(argc, argv) 53 int argc; 54 char **argv; 55 { 56 int c; 57 int line_length = 0; 58 int finger_status; 59 int wait_pid; 60 int input_count = 0; 61 struct passwd *pwd; 62 63 /* 64 * First of all, let's don't run with superuser privileges. 65 */ 66 if (getuid() == 0 || geteuid() == 0) { 67 if ((pwd = getpwnam(UNPRIV_NAME)) && pwd->pw_uid > 0) { 68 setgid(pwd->pw_gid); 69 setuid(pwd->pw_uid); 70 } else { 71 setgid(UNPRIV_UGID); 72 setuid(UNPRIV_UGID); 73 } 74 } 75 76 /* 77 * Redirect our standard input through the raw finger command. 78 */ 79 if (putenv(path)) { 80 fprintf(stderr, "%s: putenv: out of memory", argv[0]); 81 exit(1); 82 } 83 argv[0] = FINGER_PROGRAM; 84 finger_pid = pipe_stdin(argv); 85 86 /* 87 * Don't wait forever (Peter Wemm <peter@gecko.DIALix.oz.au>). 88 */ 89 signal(SIGALRM, cleanup); 90 (void) alarm(TIME_LIMIT); 91 92 /* 93 * Main filter loop. 94 */ 95 while ((c = getchar()) != EOF) { 96 if (input_count++ >= INPUT_LENGTH) { /* don't listen forever */ 97 fclose(stdin); 98 printf("\n\n Input truncated to %d bytes...\n", input_count - 1); 99 break; 100 } 101 if (c == '\n') { /* good: end of line */ 102 putchar(c); 103 line_length = 0; 104 } else { 105 if (line_length >= LINE_LENGTH) { /* force end of line */ 106 printf("\\\n"); 107 line_length = 0; 108 } 109 if (line_length == 0) { /* protect left margin */ 110 putchar(' '); 111 line_length++; 112 } 113 if (isascii(c) && (isprint(c) || isspace(c))) { /* text */ 114 if (c == '\\') { 115 putchar(c); 116 line_length++; 117 } 118 putchar(c); 119 line_length++; 120 } else { /* quote all other thash */ 121 printf("\\%03o", c & 0377); 122 line_length += 4; 123 } 124 } 125 } 126 127 /* 128 * Wait until the finger child process has terminated and account for its 129 * exit status. Which will always be zero on most systems. 130 */ 131 while ((wait_pid = wait(&finger_status)) != -1 && wait_pid != finger_pid) 132 /* void */ ; 133 return (wait_pid != finger_pid || finger_status != 0); 134 } 135 136 /* perror_exit - report system error text and terminate */ 137 138 void perror_exit(text) 139 char *text; 140 { 141 perror(text); 142 exit(1); 143 } 144 145 /* pipe_stdin - pipe stdin through program (from my ANSI to OLD C converter) */ 146 147 int pipe_stdin(argv) 148 char **argv; 149 { 150 int pipefds[2]; 151 int pid; 152 int i; 153 struct stat st; 154 155 /* 156 * The code that sets up the pipe requires that file descriptors 0,1,2 157 * are already open. All kinds of mysterious things will happen if that 158 * is not the case. The following loops makes sure that descriptors 0,1,2 159 * are set up properly. 160 */ 161 162 for (i = 0; i < 3; i++) { 163 if (fstat(i, &st) == -1 && open("/dev/null", 2) != i) 164 perror_exit("open /dev/null"); 165 } 166 167 /* 168 * Set up the pipe that interposes the command into our standard input 169 * stream. 170 */ 171 172 if (pipe(pipefds)) 173 perror_exit("pipe"); 174 175 switch (pid = fork()) { 176 case -1: /* error */ 177 perror_exit("fork"); 178 /* NOTREACHED */ 179 case 0: /* child */ 180 (void) close(pipefds[0]); /* close reading end */ 181 (void) close(1); /* connect stdout to pipe */ 182 if (dup(pipefds[1]) != 1) 183 perror_exit("dup"); 184 (void) close(pipefds[1]); /* close redundant fd */ 185 (void) execvp(argv[0], argv); 186 perror_exit(argv[0]); 187 /* NOTREACHED */ 188 default: /* parent */ 189 (void) close(pipefds[1]); /* close writing end */ 190 (void) close(0); /* connect stdin to pipe */ 191 if (dup(pipefds[0]) != 0) 192 perror_exit("dup"); 193 (void) close(pipefds[0]); /* close redundant fd */ 194 return (pid); 195 } 196 } 197