Simulation and Instrumentation

When you ask for a step of execution to happen in angr, something has to actually perform the step. angr uses a series of engines (subclasses of the SimEngine class) to emulate the effects that of a given section of code has on an input state. The execution core of angr simply tries all the available engines in sequence, taking the first one that is able to handle the step. The following is the default list of engines, in order:

  • The failure engine kicks in when the previous step took us to some uncontinuable state
  • The syscall engine kicks in when the previous step ended in a syscall
  • The hook engine kicks in when the current address is hooked
  • The unicorn engine kicks in when the UNICORN state option is enabled and there is no symbolic data in the state
  • The VEX engine kicks in as the final fallback.

SimSuccessors

The code that actually tries all the engines in turn is project.factory.successors(state, **kwargs), which passes its arguments onto each of the engines. This function is at the heart of state.step() and simulation_manager.step(). It returns a SimSuccessors object, which we discussed briefly before. The purpose of SimSuccessors is to perform a simple categorization of the successor states, stored in various list attributes. They are:

Attribute Guard Condition Instruction Pointer Description
successors True (can be symbolic, but constrained to True) Can be symbolic (but 256 solutions or less; see unconstrained_successors). A normal, satisfiable successor state to the state processed by the engine. The instruction pointer of this state may be symbolic (i.e., a computed jump based on user input), so the state might actually represent several potential continuations of execution going forward.
unsat_successors False (can be symbolic, but constrained to False). Can be symbolic. Unsatisfiable successors. These are successors whose guard conditions can only be false (i.e., jumps that cannot be taken, or the default branch of jumps that must be taken).
flat_successors True (can be symbolic, but constrained to True). Concrete value. As noted above, states in the successors list can have symbolic instruction pointers. This is rather confusing, as elsewhere in the code (i.e., in SimEngineVEX.process, when it's time to step that state forward), we make assumptions that a single program state only represents the execution of a single spot in the code. To alleviate this, when we encounter states in successors with symbolic instruction pointers, we compute all possible concrete solutions (up to an arbitrary threshold of 256) for them, and make a copy of the state for each such solution. We call this process "flattening". These flat_successors are states, each of which has a different, concrete instruction pointer. For example, if the instruction pointer of a state in successors was X+5, where X had constraints of X > 0x800000 and X < 0x800010, we would flatten it into 16 different flat_successors states, one with an instruction pointer of 0x800006, one with 0x800007, and so on until 0x800015.
unconstrained_successors True (can be symbolic, but constrained to True). Symbolic (with more than 256 solutions). During the flattening procedure described above, if it turns out that there are more than 256 possible solutions for the instruction pointer, we assume that the instruction pointer has been overwritten with unconstrained data (i.e., a stack overflow with user data). This assumption is not sound in general. Such states are placed in unconstrained_successors and not in successors.
all_successors Anything Can be symbolic. This is successors + unsat_successors + unconstrained_successors.

Breakpoints

TODO: rewrite this to fix the narrative

Like any decent execution engine, angr supports breakpoints. This is pretty cool! A point is set as follows:

>>> import angr
>>> b = angr.Project('examples/fauxware/fauxware')

# get our state
>>> s = b.factory.entry_state()

# add a breakpoint. This breakpoint will drop into ipdb right before a memory write happens.
>>> s.inspect.b('mem_write')

# on the other hand, we can have a breakpoint trigger right *after* a memory write happens. 
# we can also have a callback function run instead of opening ipdb.
>>> def debug_func(state):
...     print "State %s is about to do a memory write!"

>>> s.inspect.b('mem_write', when=angr.BP_AFTER, action=debug_func)

# or, you can have it drop you in an embedded IPython!
>>> s.inspect.b('mem_write', when=angr.BP_AFTER, action='IPython')

There are many other places to break than a memory write. Here is the list. You can break at BP_BEFORE or BP_AFTER for each of these events.

Event type Event meaning
mem_read Memory is being read.
mem_write Memory is being written.
reg_read A register is being read.
reg_write A register is being written.
tmp_read A temp is being read.
tmp_write A temp is being written.
expr An expression is being created (i.e., a result of an arithmetic operation or a constant in the IR).
statement An IR statement is being translated.
instruction A new (native) instruction is being translated.
irsb A new basic block is being translated.
constraints New constraints are being added to the state.
exit A successor is being generated from execution.
symbolic_variable A new symbolic variable is being created.
call A call instruction is hit.
address_concretization A symbolic memory access is being resolved.

These events expose different attributes:

Event type Attribute name Attribute availability Attribute meaning
mem_read mem_read_address BP_BEFORE or BP_AFTER The address at which memory is being read.
mem_read mem_read_length BP_BEFORE or BP_AFTER The length of the memory read.
mem_read mem_read_expr BP_AFTER The expression at that address.
mem_write mem_write_address BP_BEFORE or BP_AFTER The address at which memory is being written.
mem_write mem_write_length BP_BEFORE or BP_AFTER The length of the memory write.
mem_write mem_write_expr BP_BEFORE or BP_AFTER The expression that is being written.
reg_read reg_read_offset BP_BEFORE or BP_AFTER The offset of the register being read.
reg_read reg_read_length BP_BEFORE or BP_AFTER The length of the register read.
reg_read reg_read_expr BP_AFTER The expression in the register.
reg_write reg_write_offset BP_BEFORE or BP_AFTER The offset of the register being written.
reg_write reg_write_length BP_BEFORE or BP_AFTER The length of the register write.
reg_write reg_write_expr BP_BEFORE or BP_AFTER The expression that is being written.
tmp_read tmp_read_num BP_BEFORE or BP_AFTER The number of the temp being read.
tmp_read tmp_read_expr BP_AFTER The expression of the temp.
tmp_write tmp_write_num BP_BEFORE or BP_AFTER The number of the temp written.
tmp_write tmp_write_expr BP_AFTER The expression written to the temp.
expr expr BP_AFTER The value of the expression.
statement statement BP_BEFORE or BP_AFTER The index of the IR statement (in the IR basic block).
instruction instruction BP_BEFORE or BP_AFTER The address of the native instruction.
irsb address BP_BEFORE or BP_AFTER The address of the basic block.
constraints added_constraints BP_BEFORE or BP_AFTER The list of constraint expressions being added.
call function_address BP_BEFORE or BP_AFTER The name of the function being called.
exit exit_target BP_BEFORE or BP_AFTER The expression representing the target of a SimExit.
exit exit_guard BP_BEFORE or BP_AFTER The expression representing the guard of a SimExit.
exit jumpkind BP_BEFORE or BP_AFTER The expression representing the kind of SimExit.
symbolic_variable symbolic_name BP_BEFORE or BP_AFTER The name of the symbolic variable being created. The solver engine might modify this name (by appending a unique ID and length). Check the symbolic_expr for the final symbolic expression.
symbolic_variable symbolic_size BP_BEFORE or BP_AFTER The size of the symbolic variable being created.
symbolic_variable symbolic_expr BP_AFTER The expression representing the new symbolic variable.
address_concretization address_concretization_strategy BP_BEFORE or BP_AFTER The SimConcretizationStrategy being used to resolve the address. This can be modified by the breakpoint handler to change the strategy that will be applied. If your breakpoint handler sets this to None, this strategy will be skipped.
address_concretization address_concretization_action BP_BEFORE or BP_AFTER The SimAction object being used to record the memory action.
address_concretization address_concretization_memory BP_BEFORE or BP_AFTER The SimMemory object on which the action was taken.
address_concretization address_concretization_expr BP_BEFORE or BP_AFTER The AST representing the memory index being resolved. The breakpoint handler can modify this to affect the address being resolved.
address_concretization address_concretization_add_constraints BP_BEFORE or BP_AFTER Whether or not constraints should/will be added for this read.
address_concretization address_concretization_result BP_AFTER The list of resolved memory addresses (integers). The breakpoint handler can overwrite these to effect a different resolution result.

These attributes can be accessed as members of state.inspect during the appropriate breakpoint callback to access the appropriate values. You can even modify these value to modify further uses of the values!

>>> def track_reads(state):
...     print 'Read', state.inspect.mem_read_expr, 'from', state.inspect.mem_read_address
...
>>> s.inspect.b('mem_read', when=angr.BP_AFTER, action=track_reads)

Additionally, each of these properties can be used as a keyword argument to inspect.b to make the breakpoint conditional:

# This will break before a memory write if 0x1000 is a possible value of its target expression
>>> s.inspect.b('mem_write', mem_write_address=0x1000)

# This will break before a memory write if 0x1000 is the *only* value of its target expression
>>> s.inspect.b('mem_write', mem_write_address=0x1000, mem_write_address_unique=True)

# This will break after instruction 0x8000, but only 0x1000 is a possible value of the last expression that was read from memory
>>> s.inspect.b('instruction', when=angr.BP_AFTER, instruction=0x8000, mem_read_expr=0x1000)

Cool stuff! In fact, we can even specify a function as a condition:

# this is a complex condition that could do anything! In this case, it makes sure that RAX is 0x41414141 and
# that the basic block starting at 0x8004 was executed sometime in this path's history
>>> def cond(state):
...     return state.eval(state.regs.rax, cast_to=str) == 'AAAA' and 0x8004 in state.inspect.backtrace

>>> s.inspect.b('mem_write', condition=cond)

That is some cool stuff!

results matching ""

    No results matching ""