Stack Corruptions in Software

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Stack Corruptions
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with that stack location prior to returning The answer is yes and no: yes in the sense that we need the return address to know where to return to, and no because we don t explicitly pop it from the stack When the ret instruction is executed, the return address is popped from the stack and control is transferred to that location so that execution can resume
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Top of the STACK REGISTERS INSTRUCTIONS
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0x002bffb8 0x002bffb4 0x002bffb0 Saved EBP Return address from call
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5 MEMORY CORRUPTION PART I STACKS
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Figure 57 As you can see, the stack is a very versatile data structure, and it is at the heart of thread execution in Windows It enables applications to transfer control back and forth between functions in a very structured and ordered fashion Because the compiler generates all the code that handles this control transfer (managing the stack, passing parameters, addressing local variables, and so on), developers typically do not worry too much about what actually goes on behind-the-scenes For the most part, developers should not have to worry, but some very frequent programming mistakes can cause the thread stack to become corrupt When it does, understanding how the stack is managed can mean the difference between a successful application launch and disaster In the following sections, we detail some of the most common scenarios that can lead to stack corruption and ways to apply the memory corruption detection process to get to the root cause
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The Mysterious mov edi,edi Instruction
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A function prologue is responsible for setting up the current frame As we have seen, the general structure of a function prologue sets up the base frame pointer, pushes the base frame pointer onto the stack, and reserves space for local variables Here is an example of the FindFirstFileExW function prologue:
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Memory Corruption Part I Stacks
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0:000> u kernel32!FindFirstFileExW kernel32!FindFirstFileExW: 7c80ec7d 8bff mov edi,edi Useless instruction 7c80ec7f 55 push ebp Save away old base frame pointer 7c80ec80 8bec mov ebp,esp Set up new base frame pointer 7c80ec82 81eccc020000 sub esp,0x2cc Reserve space for local variables 7c80ec88 837d0c01 cmp dword ptr [ebp+0xc],0x1 7c80ec8c a1cc36887c mov eax,[kernel32!__security_cookie (7c8836cc)] 7c80ec91 53 push ebx 7c80ec92 8945fc mov [ebp-0x4],eax
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What we have not discussed yet is the very first and mysterious mov edi,edi instruction Every function prologue begins with this seemingly useless instruction Most of the time, the mov edi,edi instruction is simply a NOP (no operation), but under certain circumstances, it might be used to enable hot patching Hot patching refers to the capability to patch running code without the hassle of first stopping the component being patched This mechanism is crucial to avoiding downtime in system availability The basic principle is that the 2-byte mov edi,edi instruction can be replaced by a jmp instruction that can execute whatever new code is required Because it is a 2-byte instruction, the only jmp instruction that will actually fit is a short jmp, which enables a jump of 127 bytes in either direction This is typically not enough because chances are that you would jump to locations where existing code is already located To bypass this limitation, we have to look at the instructions preceding the mov edi,edi instruction:
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0:000> u kernel32!FindFirstFileExW-9 kernel32!OpenMutexW+a6: 7c80ec74 33c0 xor eax,eax 7c80ec76 eb98 jmp kernel32!OpenMutexW+0xad (7c80ec10) 7c80ec78 90 nop 7c80ec79 90 nop 7c80ec7a 90 nop 7c80ec7b 90 nop 7c80ec7c 90 nop kernel32!FindFirstFileExW: 7c80ec7d 8bff mov edi,edi
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The five bytes preceding the mov instruction are all 1-byte NOP instructions By replacing the mov edi,edi instruction with a short jump to the NOP instructions and replacing those instructions with a long jump, we can easily hot patch to a location of choice
Stack Corruptions
Stack Overruns
A stack overrun occurs when a thread indiscriminately overwrites portions of its call stack reserved for other purposes This can include, but is not limited to, overwriting the return address for a particular frame, overwriting entire frames, or even exhausting the stack completely The net effect of stack overruns ranges from crashes to unpredictable behavior and even serious security holes Stack overruns have become one of the most common attack angles for malicious software, as they can potentially allow the attacker to gain complete control of the computer on which the faulty software runs To exemplify the seriousness of stack overruns, we will look at a scenario in which a stack overrun could result in a security hole The seemingly innocent code in Listing 59 shows an application that accepts a connection string on the command line and attempts to use that connection string to establish a connection to a data source Listing 59