User Mode Debugger Internals
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Finally, the Windows kernel can send notifications when user modules are mapped into the memory This functionality is enabled by setting the KernelSymbolLoad(kls) flag in the same global variable as nt!NTGlobalFlag using the gflagsexe utility or the !gflag extension command After enabling the flag, we activate the notification by entering the sxe ld: <module> command in the kernel mode debugger The debugger is notified when the module is mapped in memory, which presents a good opportunity to debug the process loading it, from kernel mode Listing 329 uses the kls flag to detect the first instantiation of the notepadexe process This feature is very powerful to debug modules loaded in early stages of Windows start-up or when it is hard to predict which process will load the module of interest However, this notification is not sent if the module is already cached in the system memory Listing 329
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Using kls flag for detecting a user mode module mapping
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kd> !gflag +kls New NtGlobalFlag contents: 0x00040000 ksl - Enable loading of kernel debugger symbols kd> sxe ld notepad kd> g nt!DebugService2+0x10: 8050b897 cc int 3 kd> k ChildEBP RetAddr f3b7da24 8050b8d9 nt!DebugService2+0x10 f3b7da48 805d536c nt!DbgLoadImageSymbols+0x42 f3b7da98 805d5212 nt!MiLoadUserSymbols+0x169 f3b7dadc 8057bc22 nt!MiMapViewOfImageSection+0x4b6 f3b7db38 80503a0b nt!MmMapViewOfSection+0x13c f3b7db94 80588c21 nt!MmInitializeProcessAddressSpace+0x337 f3b7dce4 80588635 nt!PspCreateProcess+0x333 f3b7dd38 804df06b nt!NtCreateProcessEx+0x7e f3b7dd38 7c90eb94 nt!KiFastCallEntry+0xf8 WARNING: Frame IP not in any known module Following frames may be wrong 0013fa88 00000000 0x7c90eb94 kd> !process -1 0 PROCESS 82f5a020 SessionId: 0 Cid: 0000 Peb: 00000000 ParentCid: 0544 DirBase: 0de15000 ObjectTable: e1b12638 HandleCount: 1 Image: notepadexe
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After this overview of the mechanisms provided by the operating system to debug any running target process, one step is still required to understand how the debugger is capable of doing all its magic This section describes some of the levers used by debuggers to control the debugger target and how each lever influences the debugger target
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An exception having the code STATUS_BREAKPOINT is used all through this book, especially in this chapter, without a clear explanation of the way this exception is raised It is time to explain how the process generates this exception The x86 instruction set contains a special instruction named int 3 introduced to facilitate debugging by generating a STATUS_BREAKPOINT hardware exception on the processor executing this instruction In response to the STATUS_BREAKPOINT exception, the processor executes the interrupt handler registered for the interrupt vector 3 The interrupt handler converts the hardware exception into a software exception raised at the address containing the statement The instruction is represented in the instruction stream, representation called Operation Code or opcode, by a single byte with the value 0xCC Without a debugger available, the software exception is treated as a regular exception; otherwise, the Windows operating system instructs the debugger to break right at the instruction s address The debugger uses the 0xCC opcode when setting a breakpoint To set the breakpoint, the debugger changes the protection on the memory block containing the breakpoint address so that it can write an int 3 statement at that address The old value, along with the information about the breakpoint number, is then saved in the debugger memory A breakpoint address must be the address of a valid opcode in the instruction stream, which is always the first byte of a machine language instruction A breakpoint set to any other address in the machine language instruction changes the instruction meaning, without triggering a STATUS_BREAKPOINT hardware exception when that instruction is generated Needless to say, running the application containing a wrong machine language instruction is dangerous and unpredictable The changes in memory should not be visible to the user, as those changes can influence the results of unassambling code functions Therefore, when the debugger stops, it always replaces the original memory values for each breakpoint set by the debugger before doing any kind of processing Regardless of the magic used to hide
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