## Tuesday, June 10, 2014

### Of Binary Bombs (part 6)

In the last installment (phase 5) Dr. Evil used masking and a lookup table to try and defeat any secret agent. I will continue on here with the final phase of this binary bomb: phase 6. (This isn't really the final stage - check out the secret stage)

Our input string is loaded into the edx register as usual but then there is a strange reference to a sym.node1 that gets loaded into local stack space. That makes our first order of business to find what is stored in sym.node1.

The name node1 gives a fairly blatant hint at how we should look at this memory (without the symbols, this task would be a whole lot less straightforward). The first several bytes are pretty sparse: interpreting as 32-bit values we get 0xfd (253), 0x01 (1), and then the value 0x0804b260 (this is stored in little endian). That looks like another memory address; lets see.

Same structure. 0x02d5 (725), 0x02 (2), 0x0804b254. And the pattern continues. I'll take a leap and say that we have something that looks like the following C structure:

struct list_ {
int value_;
int index_;
struct list_ *next_;
};


I'm going to walk the list for a while to collect the values (and verify the counter continues in order). That results in the following (value_,index_) pairs starting from sym.node1.

(253, 1)
(725, 2)
(301, 3)
(997, 4)
(212, 5)
(432, 6)


The list is terminated at that point with a null next_ pointer. At this point, the values of the list are known so it is appropriate to resume walking the body of sym.phase_6.

Currently, the input string is loaded into edx and the linked list is stored in a local value; next a local buffer is loaded to eax and sym.read_six_numbers is called. I described this function in phase 2 and we can expect that the local buffer will contain our six input numbers after the call. I have a guess at this point what they should be but I want to verify first to avoid any of Dr. Evil's tricks.

The remainder of this phase can be broken down into four distinct loops. They are:
1. Verify the input values
2. Collect the nodes of the above list according to the input values
3. Reorder the original list with that collection
4. Verify the resulting list
While the input verification has a nested loop it is the most straightforward of the steps: it checks that all values are unique and less than 7.

Initially, collecting nodes according to the input values is a little harder to grasp as it too is a nested loop construct but is now dealing with offsetting into structures and moving memory locations (C pointers) around.

Specifically, the commented line below walks the linked list. This is something that would have not been evident had I not understood the memory in sys.node1.

    mov eax, [edx+ecx]
lea esi, [esi]
mov esi, [esi+0x8]  ; this uses the 'next' pointer
inc ebx
cmp ebx, eax


The third step, reordering the original list, is short and looks simple enough but took me some time to fully grok. I needed to understand that the previous step was storing local copies of the nodes in the original list. From that the original list is overwirtten here in the order specified by the input.

Finally, the overwritten list is checked to ensure that the value_ elements are arranged in decreasing order.

With that final piece of information the necessary input sequence becomes clear - the solution is to provide index_ values that order the value_ members from largest to smallest.

Below is a mapping of this functionality to some C code that it may have come from.

struct list_ {
int value_, index_;
struct list_ *next;
};

void phase_6 (const char * input) {

int i = 0;
struct list_ *list = ..., *node = list;
int values[6] = {0};
struct list_ *nodes[6] = {0};

// 0x08048db8 - 0x08048e00
for (; i < 6; ++i) {
int j = i + 1;
if (values[i] > 6) explode_bomb ();
for (; j < 6; ++j)
if (values[i] == values[j])
explode_bomb ();
}

// 0x08048e02 - 0x08048e42
for (i = 0 ; i < 6; ++i) {
node = list;
while (node) {
if (node->index_ == values[i]) {
nodes[i] = node;
break;
}
node = node->next;
}
}

// 0x08048e44 - 0x08048e60
i = 1;
list = nodes[0];
node = list;
while (i <= 5) {
node->next = nodes[i];
node = node->next;
++i;
}
node->next = 0;

// 0x08048e67 - 0x08048e85
node = list;
for (i = 0; i < 5; ++i)
if (node->value_ < node->next->value_)
explode_bomb ();

}

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