Top of the STACK REGISTERS INSTRUCTIONS in Software

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Top of the STACK REGISTERS INSTRUCTIONS
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ThreadProcedure
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0x002bffb8 0x002bffb4 0x002bffb0 Saved EBP Return address from call
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ESP=0x002bffb4 EBP=0x002bffb4 ESP=0x002bffb0
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push ebp mov ebp,esp call simple!ProcA
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ProcA
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0x002bffac 0x002bffa8 0x002bffa4
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Saved EBP Reserved for local variable: iNums[2]
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ESP=0x002bffac EBP=0x002bffac ESP=0x002bff98
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push ebp mov ebp,esp call esp, 0x14
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Reserved for local variable: iNums[1] Reserved for local variable: 0x002bffa0 iNums[0] 0x002bff9c Reserved for local variable: iSum 0x002bff98 Reserved for local variable: iCount 0x002bff9c 0x002bff94 (Parameter: int* sum) 0x002bff90 0x002bff8c 3 (Parameter: int iCount) 0x002bffa0 (Parameter int* numArray)
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EAX=0x002bff9c ESP=0x002bff94 ECX=3 ESP=0x002bff90 EDX=0x002bffa0 ESP=0x002bff8c
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lea eax,[ebp-0x10] push eax mov ecx,[ebp-0x14] push ecx lea edx,[ebp-0xc] push edx
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Stack Corruptions
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I will leave it as an exercise to the reader to figure out what the stack looks like in the new frame while calling the Sum function Here is a hint: Because the parameters are passed via the stack, an offset is used in conjunction with the ebp register to access the passed-in parameters After the call has returned to the calling frame (ProcA), the stack pointer esp is set to 0x002bff98, which is also the last stack slot used prior to pushing parameters for the call to Sum How did the stack pointer get adjusted back to that position The answer to that lies in how a frame returns from a function, as you will see when we analyze the return from the ProcA function Listing 57 shows the assembly instructions right after our call to Sum Listing 57
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0100128c 8b45f0 0100128f 50 01001290 68d0100001 01001295 ff1550100001 (01001050)] 0100129b 83c408 0100129e 8be5 010012a0 5d 010012a1 c3 mov push push call add mov pop ret eax,dword ptr [ebp-10h] eax offset 05stackdesc!`string (010010d0) dword ptr [05stackdesc!_imp__printf esp,8 esp,ebp ebp
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5 MEMORY CORRUPTION PART I STACKS
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The next call instruction on line 4 shows another call, this time to the printf function This matches up well with our source code, as it tries to print out the result of the call to Sum (stored in iSum) Once again, before calling the printf function, the stack is set up for any parameters that might be needed during the call More specifically, two parameters are passed:
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A string: The sum is: %d\n The value of iSum
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Remember that parameters are always passed from right to left, so we push the value of iSum onto the stack first The first two instructions of Listing 57 show how the value of iSum is pushed onto the stack Because iSum is a local variable on the ProcA frame, it is accessed via the ebp register minus an offset of 0x10 From Figure 54, we can see that ebp-0x10 indexes the iSum local variable The last parameter that should be pushed onto the stack is the string itself, and we can see that with the push offset 05stackdesc!`string (010010d0) instruction To validate that it is in fact pushing the correct string onto the stack, we can use the da (dump ASCII) command:
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5
Memory Corruption Part I Stacks
0:001> da 0x10010d0 010010d0 Sum is: %d
This does indeed validate that the correct string is being passed After the call instruction has executed, the final few instructions in the ProcA function ensure that the stack is restored to its original state prior to the call to ProcA, as shown in Listing 58 Listing 58
0100129b 0100129e 010012a0 010012a1 83c408 8be5 5d c3 add mov pop ret esp,8 esp,ebp ebp
The first instruction adds 8 to the stack pointer esp What is the reason behind this addition Well, when the printf function returns, esp is set to the last parameter that was pushed onto the stack in preparation for the call Remember that each time a frame makes a call, we need to ensure that the stack is restored to the state prior to the call Since we pushed two parameters onto the stack in order to call printf, we need to add 8 bytes from the stack pointer esp in order to get back to the state we had prior to the call (2*4 bytes = the size of the two parameters pushed onto the stack) Once the state has been restored, we are just about ready to return from the ProcA function Since we allocated local variables in the ProcA function, the esp register is pointing to the last local variable declared on the stack As we return from the function, we need to make sure that the esp register is reset to the value that it was prior to making the call to the ProcA function The key to accomplish this is to remember what took place in the ProcA function prologue More specifically, the mov ebp,esp instruction in the prologue saved the value of the esp register into ebp To restore esp, we simply execute the mov esp,ebp instruction, as shown in Listing 58 Figure 57 shows the current state of our stack Because the ebp register is used as the base frame pointer, it is as important to restore that register as it is to restore the esp register After we have returned from the ProcA function, we want the calling function (ThreadProcedure) to be capable of using the ebp register just as it was being used prior to the call to FuncA Because the next item on our stack is the saved ebp (that is, the frame pointer of the calling function), we simply pop that value into the ebp register Finally, we can issue the ret instruction to return to the calling function But, hold on our esp register (0x002bffb0) seems to be pointing to a return address that was pushed onto the stack automatically when executing the call instruction Do we have to do anything