UNIX> chmod 0666 /blugreen/homes/plank/cs360/notes/Setuid/log_fileHowever, that is unsafe, since any of you could now edit that file, truncate it, or whatever. Thus, this isn't a good solution in an environment in which you don't trust everyone.
To combat this, one of the things that Unix provides is the ``setuid'' bit. This says to set the user id of the program being exectuted to that of the owner of the executable file. You can see whether the bit is set in the "ls -l" command (it is part of the mode in "stat"):
UNIX> cd /blugreen/homes/plank/cs360/notes/Setuid UNIX> ls -l log1 log2 -rwxr-xr-x 1 plank 24576 Sep 30 09:46 log1 -rwsr-xr-x 1 plank 24576 Sep 30 09:46 log2 UNIX>Log1 is log1.c compiled normally. Log2 is created by:
UNIX> cp log1 log2 UNIX> chmod 04755 log2This sets the setuid bit. You'll notice that since log_file is chmod()'d to 0644, log1 will fail when you call it from your account, but log2 will succeed:
(YOU MUST BE LOGGED INTO THE MACHINE "vertex" FOR THIS TO WORK -- so do "ssh vertex" first.)
UNIX> echo "$USER" booth UNIX> cd /blugreen/homes/plank/cs360/notes/Setuid UNIX> cat log_file plank Mon Feb 28 11:28:58 1994 booth Mon Feb 28 11:37:24 1994 elmore Tue Mar 8 09:45:19 1994 hamner Tue Mar 8 21:14:06 1994 UNIX> log1 Can't write log file /blugreen/homes/plank/cs360/notes/Setuid/log_file UNIX> date Fri Sep 30 10:13:04 EDT 1994 UNIX> log2 UNIX> cat log_file plank Mon Feb 28 11:28:58 1994 booth Mon Feb 28 11:37:24 1994 elmore Tue Mar 8 09:45:19 1994 hamner Tue Mar 8 21:14:06 1994 booth Fri Sep 30 10:13:10 1994 UNIX>So, this gives you a way to let other users mess with your files in a restricted way. You must be very careful when using the setuid bit, that you not give users too much access. For example, look at log3.c:
#include < stdio.h >
main(int argc, char **argv)
{
char s[1000];
FILE *f;
int i;
if (argc == 2) {
sprintf(s, "/blugreen/homes/plank/cs360/notes/Setuid/%s", argv[1]);
f = fopen(s, "w");
if (f == NULL) {
perror(s);
exit(1);
}
i = 1;
while(i > 0) {
i = fread(s, sizeof(char), 1000, stdin);
if (i > 0) {
fwrite(s, sizeof(char), i, f);
}
}
fclose(f);
}
}
This file lets a user create a file in the directory
/blugreen/homes/plank/cs360/notes/Setuid and set the contents
to standard input. The user specifies the file name. Were I to set the setuid
bit of this file, then a malignant user could for example, trash my
.cshrc file by calling "log3 ../../../.cshrc". Then
he/she could set that file to whatever he/she wanted. Thus, what looks
like a benign program is really a huge security leak.
Thus, you must be careful writing programs that make use of the
setuid bit.On our systems, the setuid bit only works if you are logged onto the machine which contains the filesystem that the executable file is on. Thus, to use the "log2" program, you must be logged into the machine "vertex", as that is the machine that contains "/blugreen/homes/plank/..." The reason for this is security. Other machines mount my filesystem "remotely", and are not trusted where setuid bits are concerned.