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
UNICORNstate option is enabled and there is no symbolic data in the state
- The VEX engine kicks in as the final fallback.
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
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.
|Attribute||Guard Condition||Instruction Pointer||Description|
||True (can be symbolic, but constrained to True)||Can be symbolic (but 256 solutions or less; see
||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.|
||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).|
||True (can be symbolic, but constrained to True).||Concrete value.||As noted above, states in the
||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
||Anything||Can be symbolic.||This is
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=angr.BP_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!