Saturday, January 10, 2015

Practical Reverse Engineering p. 36 #11

Question number 11 on page 36 of Practical Reverse Engineering is as follows:

Read the Virtual Memory chapter in Intel Software Developer Manual, Volume 3 and AMD64 Architecture Programmer’s Manual, Volume 2: System Programming. Perform a few virtual address to physical address translations yourself and verify the result with a kernel debugger. Explain how data execution prevention (DEP) works.

Converting Virtual to Physical Memory

To convert a virtual address to its physical address you must obtain the page frame number of the directory base. You add the offset to the beginning of the page address.

You can do this with a kernel debugger with the following commands. Here is an example starting with virtual address 0x00a03ffd:

kd> !process 0 0
PROCESS ffadc791 SessionId: 0 Cid: 09ad Peb: 9afcf000 ParentCid: 0394
DirBase: 08cb1000 ObjectTable: 5b74db90 TableSize: 8.
Image: test.exe

kd> !vtop 8cb1 a03ffd
Pdi 0 Pti a03
00a03ffd 07a2f000 pfn(07a2f)

Add the offset to the base, 0x07a2f000 + 0xffd = 0x07a2fffd. So virtual address 0x00a03ffd in this process points to physical address 0x07a2fffd.

Data Execution Prevention

Hardware DEP works by enabling the NX/XD/XN-bit in the CPU (depending on architecture). This allows the operating system to mark pages of memory as non-executable. There is an "executable" flag in a memory segment's page table entry (page descriptor) that allows you to modify this setting.

If the instruction pointer moves to an area marked as DEP, the processor faults and execution is passed to a given handler. A DEP access violation in kernel mode on Windows will result in an error 0xFC: ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY.

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