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editing: parse_segv.py

#!/usr/bin/python3
#
# Copyright 2009-2010  Canonical, Ltd.
# Author: Kees Cook <kees@ubuntu.com>
#
# This program is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the
# Free Software Foundation; either version 2 of the License, or (at your
# option) any later version.  See http://www.gnu.org/copyleft/gpl.html for
# the full text of the license.

"""Examine the crash files saved by apport to attempt to determine the cause
of a segfault.  Currently very very simplistic, and only finds commonly
understood situations for x86/x86_64."""

# TODO: Address following pylint complaints
# pylint: disable=invalid-name,missing-class-docstring,missing-function-docstring

import logging
import re
import sys

class ParseSegv:
    def __init__(self, registers, disassembly, maps):
        self.regs = self.parse_regs(registers)
        self.sp = None
        for reg in ("rsp", "esp"):
            if reg in self.regs:
                self.sp = self.regs[reg]

(self.line, self.pc, self.insn, self.src, self.dest) = self.parse_disassembly(
            disassembly
        )

self.stack_vma = None
        self.maps = self.parse_maps(maps)

def find_vma(self, addr):
        for vma in self.maps:
            if vma["start"] <= addr < vma["end"]:
                return vma
        return None

def parse_maps(self, maps_str):
        maps = []
        for line in maps_str.splitlines():
            items = line.strip().split()
            if len(items) < 4:
                raise ValueError(f"Cannot parse maps line: {line.strip()}")
            span, perms = items[0:2]
            if len(items) == 5:
                name = None
            else:
                name = items[5]
            start, end = [int(x, 16) for x in span.split("-")]
            if name == "[stack]":
                self.stack_vma = len(maps)
            maps.append({"start": start, "end": end, "perms": perms, "name": name})
            logging.debug(
                "start: %s, end: %s, perms: %s, name: %s", start, end, perms, name
            )
        return maps

@staticmethod
    def parse_regs(reg_str):
        regs = {}
        for line in reg_str.splitlines():
            reg, hexvalue = line.split()[0:2]
            regs[reg] = int(hexvalue, 16)
            logging.debug("%s:0x%08x", reg, regs[reg])
        return regs

def parse_disassembly(self, disassembly):
        # TODO: Split into smaller functions/methods
        # pylint: disable=too-many-branches
        if not self.regs:
            raise ValueError("Registers not loaded yet!?")
        lines = disassembly.splitlines()
        # Throw away possible 'Dump' gdb report line
        if len(lines) > 0 and lines[0].startswith("Dump"):
            lines.pop(0)
        if len(lines) < 1:
            raise ValueError("Failed to load empty disassembly")
        line = lines[0].strip()
        # Drop GDB 7.1's leading $pc mark
        if line.startswith("=>"):
            line = line[2:].strip()
        logging.debug(line)
        pc_str = line.split()[0]
        if pc_str.startswith("0x"):
            pc = int(pc_str.split(":")[0], 16)
        else:
            # Could not identify this instruction line
            raise ValueError(
                f'Could not parse PC "{pc_str}" from disassembly line: {line}'
            )
        logging.debug("pc: 0x%08x", pc)

full_insn_str = line.split(":", 1)[1].strip()
        # Handle invalid memory
        if "Cannot access memory at address" in full_insn_str or (
            full_insn_str == "" and len(lines) == 1
        ):
            return line, pc, None, None, None
        # Handle wrapped lines
        if full_insn_str == "" and lines[1].startswith(" "):
            line = f"{line} {lines[1].strip()}"
            full_insn_str = line.split(":", 1)[1].strip()

insn_parts = full_insn_str.split()
        # Drop call target names "call   0xb7a805af <_Unwind_Find_FDE@plt+111>"
        if insn_parts[-1].endswith(">") and insn_parts[-1].startswith("<"):
            insn_parts.pop(-1)
        # Attempt to find arguments
        args_str = ""
        if len(insn_parts) > 1:
            args_str = insn_parts.pop(-1)
        # Assume remainder is the insn itself
        insn = " ".join(insn_parts)
        logging.debug("insn: %s", insn)

args = []
        src = None
        dest = None
        if args_str == "":
            # Could not find insn args
            args = None
        else:
            logging.debug('args: "%s"', args_str)

for m in re.finditer(r"([^,$]*(\(:?[^$]+\))*)", args_str):
                if len(m.group(0)):
                    args.append(m.group(0))
            if len(args) > 0:
                src = args[0]
                logging.debug("src: %s", src)
            if len(args) > 1:
                dest = args[1]
                logging.debug("dest: %s", dest)

# Set up possible implicit memory destinations (stack actions)
        if insn in {"push", "pop", "pushl", "popl", "call", "callq", "ret", "retq"}:
            for reg in ("rsp", "esp"):
                if reg in self.regs:
                    dest = f"(%{reg})"
                    break

return line, pc, insn, src, dest

def validate_vma(self, perm, addr, name):
        perm_name = {
            "x": ["executable", "executing"],
            "r": ["readable", "reading"],
            "w": ["writable", "writing"],
        }
        vma = self.find_vma(addr)
        if vma is None:
            alarmist = "unknown"
            if addr < 65536:
                alarmist = "NULL"
            return (
                False,
                f"{name} (0x{addr:08x}) not located in a known VMA region"
                f" (needed {perm_name[perm][0]} region)!",
                f"{perm_name[perm][1]} {alarmist} VMA",
            )
        if perm not in vma["perms"]:
            alarmist = ""
            if perm == "x":
                if "w" in vma["perms"]:
                    alarmist = "writable "
                else:
                    alarmist = "non-writable "
            short = f"{perm_name[perm][1]} {alarmist}VMA {vma['name']}"

return (
                False,
                f"{name} (0x{addr:08x}) in non-{perm_name[perm][0]} VMA"
                f" region: 0x{vma['start']:08x}-0x{vma['end']:08x}"
                f" {vma['perms']} {vma['name']}",
                short,
            )

return (True, f"{name} (0x{addr:08x}) ok", f"{perm_name[perm][1]} ok")

def register_value(self, reg):
        reg_orig = reg

mask = 0
        if reg.startswith("%"):
            reg = reg[1:]
        if reg in self.regs:
            return self.regs[reg]

if len(reg) == 2 and reg.endswith("l"):
            mask |= 0xFF00
            reg = f"{reg[0]}x"
        if reg in self.regs:
            return self.regs[reg] & ~mask

if len(reg) == 2 and reg.endswith("x"):
            mask |= 0xFFFF0000
            reg = f"e{reg}"
        if reg in self.regs:
            return self.regs[reg] & ~mask

if len(reg) == 3 and reg.startswith("e"):
            mask |= 0xFFFFFFFF00000000
            reg = f"r{reg[1:]}"
        if reg in self.regs:
            return self.regs[reg] & ~mask
        raise ValueError(f"Could not resolve register '{reg_orig}'")

def calculate_arg(self, arg):
        # TODO: Split into smaller functions/methods
        # pylint: disable=too-many-branches

# Check for and pre-remove segment offset
        segment = 0
        if arg.startswith("%") and ":" in arg:
            parts = arg.split(":", 1)
            segment = self.regs[parts[0][1:]]
            arg = parts[1]

# Handle standard offsets
        parts = arg.split("(")
        offset = parts[0]
        # Handle negative signs
        sign = 1
        if offset.startswith("-"):
            sign = -1
            offset = offset[1:]
        # Skip call target dereferences
        if offset.startswith("*"):
            offset = offset[1:]
        if len(offset) > 0:
            if offset.startswith("%"):
                # Handle the *%REG case
                add = self.regs[offset[1:]]
            else:
                if not offset.startswith("0x"):
                    raise ValueError(f"Unknown offset literal: {parts[0]}")
                add = int(offset[2:], 16) * sign
        else:
            add = 0

def _reg_val(self, text, val=0):
            if text.startswith("%"):
                val = self.regs[text[1:]]
            elif text == "":
                val = 0
            else:
                val = int(text)
            return val

# (%ebx, %ecx, 4) style
        value = 0
        if len(parts) > 1:
            parens = parts[1][0:-1]
            reg_list = parens.split(",")

base = 0
            if len(reg_list) > 0:
                base = _reg_val(self, reg_list[0], base)
            index = 0
            if len(reg_list) > 1:
                index = _reg_val(self, reg_list[1], index)
            scale = 1
            if len(reg_list) > 2:
                scale = _reg_val(self, reg_list[2], scale)
            value = base + index * scale

value = segment + value + add
        if "esp" in self.regs:
            # 32bit
            return value % 0x100000000
        # 64bit
        return value % 0x10000000000000000

def report(self):
        # TODO: Split into smaller functions/methods
        # pylint: disable=too-many-branches,too-many-statements
        understood = False
        reason = []
        details = [f"Segfault happened at: {self.line}"]

# Verify PC is in an executable region
        valid, out, short = self.validate_vma("x", self.pc, "PC")
        details.append(out)
        if not valid:
            reason.append(short)
            understood = True

if self.insn in {"lea", "leal"}:
            # Short-circuit for instructions that do not cause vma access
            details.append(f"insn ({self.insn}) does not access VMA")
        else:
            # Verify source is readable
            if self.src:
                if (
                    ":" not in self.src
                    and (self.src[0] in {"%", "$", "*"})
                    and not self.src.startswith("*%")
                ):
                    details.append(f'source "{self.src}" ok')
                else:
                    addr = self.calculate_arg(self.src)
                    valid, out, short = self.validate_vma(
                        "r", addr, f'source "{self.src}"'
                    )
                    details.append(out)
                    if not valid:
                        reason.append(short)
                        understood = True

# Verify destination is writable
            if self.dest:
                if ":" not in self.dest and (self.dest[0] in {"%", "$", "*"}):
                    details.append(f'destination "{self.dest}" ok')
                else:
                    addr = self.calculate_arg(self.dest)
                    valid, out, short = self.validate_vma(
                        "w", addr, f'destination "{self.dest}"'
                    )
                    details.append(out)
                    if not valid:
                        reason.append(short)
                        understood = True

# Handle I/O port operations
        if self.insn in {"out", "in"} and not understood:
            msg = (
                f"disallowed I/O port operation"
                f" on port {self.register_value(self.src)}"
            )
            reason.append(msg)
            details.append(msg)
            understood = True

# Note position of SP with regard to "[stack]" VMA
        if self.sp is not None:
            if self.stack_vma is not None:
                if self.sp < self.maps[self.stack_vma]["start"]:
                    details.append("Stack memory exhausted (SP below stack segment)")
                if self.sp >= self.maps[self.stack_vma]["end"]:
                    details.append("Stack pointer not within stack segment")
            if not understood:
                valid, out, short = self.validate_vma("r", self.sp, "SP")
                details.append(out)
                if not valid:
                    reason.append(short)
                    understood = True

if not understood:
            vma = self.find_vma(self.pc)
            msg = "Reason could not be automatically determined."
            if vma and (vma["name"] == "[vdso]" or vma["name"] == "[vsyscall]"):
                msg += " (Unhandled exception in kernel code?)"
            reason.append(msg)
            details.append(msg)
        return understood, "\n".join(reason), "\n".join(details)

def add_info(report):
    # Only interested in segmentation faults...
    if report.get("Signal", "0") != "11":
        return

needed = ["Signal", "Architecture", "Disassembly", "ProcMaps", "Registers"]
    for field in needed:
        if field not in report:
            report["SegvAnalysis"] = f'Skipped: missing required field "{field}"'
            return

# Only run on segv for x86 and x86_64...
    if not report["Architecture"] in {"i386", "amd64"}:
        return

try:
        segv = ParseSegv(report["Registers"], report["Disassembly"], report["ProcMaps"])
        understood, reason, details = segv.report()
        if understood:
            report["SegvReason"] = reason
        report["SegvAnalysis"] = details
    except Exception as error:  # pylint: disable=broad-except
        report["SegvAnalysis"] = f"Failure: {str(error)}"

# pylint: disable-next=missing-function-docstring
def main():
    if len(sys.argv) != 4 or sys.argv[1] in {"-h", "--help"}:
        print("To run self-test, run without any arguments (or with -v)")
        print("To do stand-alone crash parsing:")
        print(f"  Usage: {sys.argv[0]} Registers.txt Disassembly.txt ProcMaps.txt")
        sys.exit(0)

with open(sys.argv[1], encoding="utf-8") as registers_file:
        registers = registers_file.read()
    with open(sys.argv[2], encoding="utf-8") as disassembly_file:
        disassembly = disassembly_file.read()
    with open(sys.argv[3], encoding="utf-8") as maps_file:
        maps = maps_file.read()
    segv = ParseSegv(registers, disassembly, maps)
    understood, reason, details = segv.report()
    print(f"{reason}\n\n{details}")
    rc = 0
    if not understood:
        rc = 1
    sys.exit(rc)

if __name__ == "__main__":
    main()

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