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
cleanup(sig)45 void cleanup(sig)
46 int sig;
47 {
48 kill(finger_pid, SIGKILL);
49 exit(0);
50 }
51
main(int argc,char ** argv)52 main(int argc, char **argv)
53 {
54 int c;
55 int line_length = 0;
56 int finger_status;
57 int wait_pid;
58 int input_count = 0;
59 struct passwd *pwd;
60
61 /*
62 * First of all, let's don't run with superuser privileges.
63 */
64 if (getuid() == 0 || geteuid() == 0) {
65 if ((pwd = getpwnam(UNPRIV_NAME)) && pwd->pw_uid > 0) {
66 setgid(pwd->pw_gid);
67 setuid(pwd->pw_uid);
68 } else {
69 setgid(UNPRIV_UGID);
70 setuid(UNPRIV_UGID);
71 }
72 }
73
74 /*
75 * Redirect our standard input through the raw finger command.
76 */
77 if (putenv(path)) {
78 fprintf(stderr, "%s: putenv: out of memory", argv[0]);
79 exit(1);
80 }
81 argv[0] = FINGER_PROGRAM;
82 finger_pid = pipe_stdin(argv);
83
84 /*
85 * Don't wait forever (Peter Wemm <peter@gecko.DIALix.oz.au>).
86 */
87 signal(SIGALRM, cleanup);
88 (void) alarm(TIME_LIMIT);
89
90 /*
91 * Main filter loop.
92 */
93 while ((c = getchar()) != EOF) {
94 if (input_count++ >= INPUT_LENGTH) { /* don't listen forever */
95 fclose(stdin);
96 printf("\n\n Input truncated to %d bytes...\n", input_count - 1);
97 break;
98 }
99 if (c == '\n') { /* good: end of line */
100 putchar(c);
101 line_length = 0;
102 } else {
103 if (line_length >= LINE_LENGTH) { /* force end of line */
104 printf("\\\n");
105 line_length = 0;
106 }
107 if (line_length == 0) { /* protect left margin */
108 putchar(' ');
109 line_length++;
110 }
111 if (isascii(c) && (isprint(c) || isspace(c))) { /* text */
112 if (c == '\\') {
113 putchar(c);
114 line_length++;
115 }
116 putchar(c);
117 line_length++;
118 } else { /* quote all other thash */
119 printf("\\%03o", c & 0377);
120 line_length += 4;
121 }
122 }
123 }
124
125 /*
126 * Wait until the finger child process has terminated and account for its
127 * exit status. Which will always be zero on most systems.
128 */
129 while ((wait_pid = wait(&finger_status)) != -1 && wait_pid != finger_pid)
130 /* void */ ;
131 return (wait_pid != finger_pid || finger_status != 0);
132 }
133
134 /* perror_exit - report system error text and terminate */
135
perror_exit(char * text)136 void perror_exit(char *text)
137 {
138 perror(text);
139 exit(1);
140 }
141
142 /* pipe_stdin - pipe stdin through program (from my ANSI to OLD C converter) */
143
pipe_stdin(char ** argv)144 int pipe_stdin(char **argv)
145 {
146 int pipefds[2];
147 int pid;
148 int i;
149 struct stat st;
150
151 /*
152 * The code that sets up the pipe requires that file descriptors 0,1,2
153 * are already open. All kinds of mysterious things will happen if that
154 * is not the case. The following loops makes sure that descriptors 0,1,2
155 * are set up properly.
156 */
157
158 for (i = 0; i < 3; i++) {
159 if (fstat(i, &st) == -1 && open("/dev/null", 2) != i)
160 perror_exit("open /dev/null");
161 }
162
163 /*
164 * Set up the pipe that interposes the command into our standard input
165 * stream.
166 */
167
168 if (pipe(pipefds))
169 perror_exit("pipe");
170
171 switch (pid = fork()) {
172 case -1: /* error */
173 perror_exit("fork");
174 /* NOTREACHED */
175 case 0: /* child */
176 (void) close(pipefds[0]); /* close reading end */
177 (void) close(1); /* connect stdout to pipe */
178 if (dup(pipefds[1]) != 1)
179 perror_exit("dup");
180 (void) close(pipefds[1]); /* close redundant fd */
181 (void) execvp(argv[0], argv);
182 perror_exit(argv[0]);
183 /* NOTREACHED */
184 default: /* parent */
185 (void) close(pipefds[1]); /* close writing end */
186 (void) close(0); /* connect stdin to pipe */
187 if (dup(pipefds[0]) != 0)
188 perror_exit("dup");
189 (void) close(pipefds[0]); /* close redundant fd */
190 return (pid);
191 }
192 }
193