Tuesday, April 8, 2014

How to make sure a process doesn’t access unauthorized part of stack


Discuss how would you make sure that a process doesn’t access an unauthorized part of the stack


As with any ambiguously worded interview question, it may help to probe the interviewer to understand what specifically you’re intended to solve. Are you trying to prevent code that has overflowed a buffer from compromising the execution by overwriting stack values? Are you trying to maintain some form of thread-specific isolation between threads? Is the code of interest native code like C++ or running under a virtual machine like Java?
Remember that, in a multi-threaded environment, there can be multiple stacks in a process.

Lets understand what is stack and heap in memory?
Stack vs heap
The stack is the memory set aside as scratch space for a thread of execution. When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data. When that function returns, the block becomes unused and can be used the next time a function is called. The stack is always reserved in a LIFO order; the most recently reserved block is always the next block to be freed. This makes it really simple to keep track of the stack; freeing a block from the stack is nothing more than adjusting one pointer.
The heap is memory set aside for dynamic allocation. Unlike the stack, there's no enforced pattern to the allocation and deallocation of blocks from the heap; you can allocate a block at any time and free it at any time. This makes it much more complex to keep track of which parts of the heap are allocated or free at any given time; there are many custom heap allocators available to tune heap performance for different usage patterns.

Native Code
One threat to the stack is malicious program input, which can overflow a buffer and overwrite stack pointers, thus circumventing the intended execution of the program.
If the interviewer is looking for a simple method to reduce the risk of buffer overflows in native code, modern compilers provide this sort of stack protection through a command line option. With Microsoft’s CL, you just pass /GS to the compiler. With GCC, you can use -fstack-protector-all.
For more complex schemes, you could set individual permissions on the range of memory pages representing the stack section you care about. In the Win32 API, you’d use the VirtualProtect API to mark the page PAGE_READONLY or PAGE_NOACCESS. This will cause the code accessing the region to go through an exception on access to the specific section of the stack .
Alternately, you could use the HW Debug Registers (DRs) to set a read or write breakpoint on the specific memory addresses of interest. A separate process could be used to debug the process of interest, catch the HW exception that would be generated if this section of the stack were accessed.
However, it’s very important to note that under normal circumstances, threads and processes are not means of access control. Nothing can prevent native code from writing anywhere within the address space of its process, including to the stack. Specifically, there is nothing to prevent malicious code in the process from calling VirtualProtect and marking the stack sections of interest PAGE_EXECUTE_READWRITE. Equally so, nothing prevents code from zeroing out the HW debug registers, eliminating your breakpoints. In summary, nothing can fully prevent native code from accessing memory addresses, including the stack, within its own process space.

A final option is to consider requiring this code that should be “sandboxed” to run in a managed language like Java or C#/.NET. By default, the virtual machines running managed code in these languages make it impossible to gain complete access to the stack from within the process.
One can use further security features of the runtimes to prevent the code from spawning additional processes or running “unsafe” code to inspect the stack. With .NET, for example, you can use Code Access Security (CAS) or appdomain permissions to control such execution.



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