Python bindings for Valgrind's VEX IR.
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ltfish Lifter: Do not extend a basic block if the lifter intentionally ends …
…it with a NoDecode. (#146)

* Lifter: Do not extend a basic block if the lifter intentionally ends it with a NoDecode.

The rationale behind is that we want to differentiate between NoDecode
that are caused by unsupported instructions and NoDecode caused by
decodeable instructions. For example, `ud2` is a valid instruction on X86
and AMD64, but it means "undefined instruction". On AMD64, VEX will create
a basic block that has an instruction of two bytes (which is the size of
the ud2 instruction), but with the `next` of the basic block pointing to
that instruction. In our existing implementation, we will ignore ud2 and
keep lifting, which is an incorrect behavior.

This commit will detect these intentional NoDecode cases. In such cases,
the Lifter will return a basic block that terminates after the
"undecodeable" instruction, with NoDecode as the jumpkind.

* Add a test case.
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README.md

PyVEX

Installing PyVEX

PyVEX can be pip-installed:

pip install pyvex

Citing PyVEX

If you use PyVEX in an academic work, please cite the paper for which it was developed:

@article{shoshitaishvili2015firmalice,
  title={Firmalice - Automatic Detection of Authentication Bypass Vulnerabilities in Binary Firmware},
  author={Shoshitaishvili, Yan and Wang, Ruoyu and Hauser, Christophe and Kruegel, Christopher and Vigna, Giovanni},
  booktitle={NDSS},
  year={2015}
}

Using PyVEX

PyVEX exposes VEX into Python. To understand VEX, read the "VEX Intermediate Representation" section below.

import pyvex
import archinfo

# translate an AMD64 basic block (of nops) at 0x400400 into VEX
irsb = pyvex.IRSB("\x90\x90\x90\x90\x90", 0x400400, archinfo.ArchAMD64())

# pretty-print the basic block
irsb.pp()

# this is the IR Expression of the jump target of the unconditional exit at the end of the basic block
print irsb.next

# this is the type of the unconditional exit (i.e., a call, ret, syscall, etc)
print irsb.jumpkind

# you can also pretty-print it
irsb.next.pp()

# iterate through each statement and print all the statements
for stmt in irsb.statements:
    stmt.pp()

# pretty-print the IR expression representing the data, and the *type* of that IR expression written by every store statement
import pyvex
for stmt in irsb.statements:
    if isinstance(stmt, pyvex.IRStmt.Store):
        print "Data:",
        stmt.data.pp()
        print ""

        print "Type:",
        print stmt.data.result_type
        print ""

# pretty-print the condition and jump target of every conditional exit from the basic block
for stmt in irsb.statements:
    if isinstance(stmt, pyvex.IRStmt.Exit):
        print "Condition:",
        stmt.guard.pp()
        print ""

        print "Target:",
        stmt.dst.pp()
        print ""

# these are the types of every temp in the IRSB
print irsb.tyenv.types

# here is one way to get the type of temp 0
print irsb.tyenv.types[0]

Keep in mind that this is a syntactic respresentation of a basic block. That is, it'll tell you what the block means, but you don't have any context to say, for example, what actual data is written by a store instruction.

VEX Intermediate Representation

To deal with widely diverse architectures, it is useful to carry out analyses on an intermediate representation. An IR abstracts away several architecture differences when dealing with different architectures, allowing a single analysis to be run on all of them:

  • Register names. The quantity and names of registers differ between architectures, but modern CPU designs hold to a common theme: each CPU contains several general purpose registers, a register to hold the stack pointer, a set of registers to store condition flags, and so forth. The IR provides a consistent, abstracted interface to registers on different platforms. Specifically, VEX models the registers as a separate memory space, with integer offsets (i.e., AMD64's rax is stored starting at address 16 in this memory space).
  • Memory access. Different architectures access memory in different ways. For example, ARM can access memory in both little-endian and big-endian modes. The IR must abstracts away these differences.
  • Memory segmentation. Some architectures, such as x86, support memory segmentation through the use of special segment registers. The IR understands such memory access mechanisms.
  • Instruction side-effects. Most instructions have side-effects. For example, most operations in Thumb mode on ARM update the condition flags, and stack push/pop instructions update the stack pointer. Tracking these side-effects in an ad hoc manner in the analysis would be crazy, so the IR makes these effects explicit.

There are lots of choices for an IR. We use VEX, since the uplifting of binary code into VEX is quite well supported. VEX is an architecture-agnostic, side-effects-free representation of a number of target machine languages. It abstracts machine code into a representation designed to make program analysis easier. This representation has four main classes of objects:

  • Expressions. IR Expressions represent a calculated or constant value. This includes memory loads, register reads, and results of arithmetic operations.
  • Operations. IR Operations describe a modification of IR Expressions. This includes integer arithmetic, floating-point arithmetic, bit operations, and so forth. An IR Operation applied to IR Expressions yields an IR Expression as a result.
  • Temporary variables. VEX uses temporary variables as internal registers: IR Expressions are stored in temporary variables between use. The content of a temporary variable can be retrieved using an IR Expression. These temporaries are numbered, starting at t0. These temporaries are strongly typed (i.e., "64-bit integer" or "32-bit float").
  • Statements. IR Statements model changes in the state of the target machine, such as the effect of memory stores and register writes. IR Statements use IR Expressions for values they may need. For example, a memory store IR Statement uses an IR Expression for the target address of the write, and another IR Expression for the content.
  • Blocks. An IR Block is a collection of IR Statements, representing an extended basic block (termed "IR Super Block" or "IRSB") in the target architecture. A block can have several exits. For conditional exits from the middle of a basic block, a special Exit IR Statement is used. An IR Expression is used to represent the target of the unconditional exit at the end of the block.

VEX IR is actually quite well documented in the libvex_ir.h file (https://github.com/angr/vex/blob/dev/pub/libvex_ir.h) in the VEX repository. For the lazy, we'll detail some parts of VEX that you'll likely interact with fairly frequently. To begin with, here are some IR Expressions:

IR Expression Evaluated Value VEX Output Example
Constant A constant value. 0x4:I32
Read Temp The value stored in a VEX temporary variable. RdTmp(t10)
Get Register The value stored in a register. GET:I32(16)
Load Memory The value stored at a memory address, with the address specified by another IR Expression. LDle:I32 / LDbe:I64
Operation A result of a specified IR Operation, applied to specified IR Expression arguments. Add32
If-Then-Else If a given IR Expression evaluates to 0, return one IR Expression. Otherwise, return another. ITE
Helper Function VEX uses C helper functions for certain operations, such as computing the conditional flags registers of certain architectures. These functions return IR Expressions. function_name()

These expressions are then, in turn, used in IR Statements. Here are some common ones:

IR Statement Meaning VEX Output Example
Write Temp Set a VEX temporary variable to the value of the given IR Expression. WrTmp(t1) = (IR Expression)
Put Register Update a register with the value of the given IR Expression. PUT(16) = (IR Expression)
Store Memory Update a location in memory, given as an IR Expression, with a value, also given as an IR Expression. STle(0x1000) = (IR Expression)
Exit A conditional exit from a basic block, with the jump target specified by an IR Expression. The condition is specified by an IR Expression. if (condition) goto (Boring) 0x4000A00:I32

An example of an IR translation, on ARM, is produced below. In the example, the subtraction operation is translated into a single IR block comprising 5 IR Statements, each of which contains at least one IR Expression (although, in real life, an IR block would typically consist of more than one instruction). Register names are translated into numerical indices given to the GET Expression and PUT Statement. The astute reader will observe that the actual subtraction is modeled by the first 4 IR Statements of the block, and the incrementing of the program counter to point to the next instruction (which, in this case, is located at 0x59FC8) is modeled by the last statement.

The following ARM instruction:

subs R2, R2, #8

Becomes this VEX IR:

t0 = GET:I32(16)
t1 = 0x8:I32
t3 = Sub32(t0,t1)
PUT(16) = t3
PUT(68) = 0x59FC8:I32

Cool stuff!