Remove attacker-controlled error messages

Creates exceptions in the server code to be shared with the client via an
identifying exit code. These exceptions are then reconstructed in the
client.

Refs #456 but does not completely fix it. Unexpected exceptions and
progress descriptions are still passed in Containers.

dangerzone/conversion/doc_to_pixels.py

@@ -13,10 +13,11 @@ import os
 import re
 import shutil
 import sys
-from typing import Dict, Optional
+from typing import Dict, List, Optional
 
 import magic
 
+from . import errors
 from .common import DangerzoneConverter, running_on_qubes
 
 
@@ -138,7 +139,7 @@ class DocumentToPixels(DangerzoneConverter):
 
         # Validate MIME type
         if mime_type not in conversions:
-            raise ValueError("The document format is not supported")
+            raise errors.DocFormatUnsupported()
 
         # Temporary fix for the HWPX format
         # Should be removed after new release of `file' (current release 5.44)
@@ -167,7 +168,7 @@ class DocumentToPixels(DangerzoneConverter):
             #     https://github.com/freedomofpress/dangerzone/issues/494
             #     https://github.com/freedomofpress/dangerzone/issues/498
             if libreoffice_ext == "h2orestart.oxt" and running_on_qubes():
-                raise ValueError("HWP / HWPX formats are not supported in Qubes")
+                raise errors.DocFormatUnsupportedHWPQubes()
             if libreoffice_ext:
                 await self.install_libreoffice_ext(libreoffice_ext)
             self.update_progress("Converting to PDF using LibreOffice")
@@ -196,7 +197,7 @@ class DocumentToPixels(DangerzoneConverter):
             #
             #     https://github.com/freedomofpress/dangerzone/issues/494
             if not os.path.exists(pdf_filename):
-                raise ValueError("Conversion to PDF with LibreOffice failed")
+                raise errors.LibreofficeFailure()
         elif conversion["type"] == "convert":
             self.update_progress("Converting to PDF using GraphicsMagick")
             args = [
@@ -216,7 +217,7 @@ class DocumentToPixels(DangerzoneConverter):
             )
             pdf_filename = "/tmp/input_file.pdf"
         else:
-            raise ValueError(
+            raise errors.InvalidGMConversion(
                 f"Invalid conversion type {conversion['type']} for MIME type {mime_type}"
             )
         self.percentage += 3
@@ -236,7 +237,7 @@ class DocumentToPixels(DangerzoneConverter):
         if search is not None:
             num_pages: int = int(search.group(1))
         else:
-            raise ValueError("Number of pages could not be extracted from PDF")
+            raise errors.NoPageCountException()
 
         # Get a more precise timeout, based on the number of pages
         timeout = self.calculate_timeout(size, num_pages)
@@ -283,7 +284,7 @@ class DocumentToPixels(DangerzoneConverter):
                 # Read the header
                 header = f.readline().decode().strip()
                 if header != "P6":
-                    raise ValueError("Invalid PPM header")
+                    raise errors.PDFtoPPMInvalidHeader()
 
                 # Save the width and height
                 dims = f.readline().decode().strip()
@@ -296,7 +297,7 @@ class DocumentToPixels(DangerzoneConverter):
                 maxval = int(f.readline().decode().strip())
                 # Check that the depth is 8
                 if maxval != 255:
-                    raise ValueError("Invalid PPM depth")
+                    raise errors.PDFtoPPMInvalidDepth()
 
                 data = f.read()
 
@@ -370,10 +371,11 @@ async def main() -> int:
     try:
         await converter.convert()
         error_code = 0  # Success!
-    except (RuntimeError, TimeoutError, ValueError) as e:
+    except errors.ConversionException as e:  # Expected Errors
+        error_code = e.error_code
+    except (RuntimeError, TimeoutError, ValueError) as e:  # Unexpected Errors
         converter.update_progress(str(e), error=True)
         error_code = 1
-
     if not running_on_qubes():
         # Write debug information (containers version)
         with open("/tmp/dangerzone/captured_output.txt", "wb") as container_log:

dangerzone/conversion/doc_to_pixels_qubes_wrapper.py

@@ -6,6 +6,7 @@ import tempfile
 from pathlib import Path
 from typing import Optional, TextIO
 
+from . import errors
 from .doc_to_pixels import DocumentToPixels
 
 
@@ -67,8 +68,13 @@ async def main() -> None:
     with open("/tmp/input_file", "wb") as f:
         f.write(data)
 
-    converter = DocumentToPixels()
-    await converter.convert()
+    try:
+        converter = DocumentToPixels()
+        await converter.convert()
+    except errors.ConversionException as e:
+        sys.exit(e.error_code)
+    except (RuntimeError, TimeoutError, ValueError) as e:
+        sys.exit(1)
 
     num_pages = len(list(out_dir.glob("*.rgb")))
     await write_int(num_pages)

dangerzone/conversion/errors.py

@@ -0,0 +1,73 @@
+from typing import List, Optional, Type
+
+
+class ConversionException(Exception):
+    error_message = "Unspecified error"
+    error_code = -1
+
+    def __init__(self, error_message: Optional[str] = None) -> None:
+        if error_message:
+            self.error_message = error_message
+        super().__init__(self.error_message)
+
+    @classmethod
+    def get_subclasses(cls) -> List[Type["ConversionException"]]:
+        subclasses = [cls]
+        for subclass in cls.__subclasses__():
+            subclasses += subclass.get_subclasses()
+        return subclasses
+
+
+class DocFormatUnsupported(ConversionException):
+    error_code = 10
+    error_message = "The document format is not supported"
+
+
+class DocFormatUnsupportedHWPQubes(DocFormatUnsupported):
+    error_code = 16
+    error_message = "HWP / HWPX formats are not supported in Qubes"
+
+
+class LibreofficeFailure(ConversionException):
+    error_code = 20
+    error_message = "Conversion to PDF with LibreOffice failed"
+
+
+class InvalidGMConversion(ConversionException):
+    error_code = 30
+    error_message = "Invalid conversion (Graphics Magic)"
+
+    def __init__(self, error_message: str) -> None:
+        super(error_message)
+
+
+class PagesException(ConversionException):
+    error_code = 40
+
+
+class NoPageCountException(PagesException):
+    error_code = 41
+    error_message = "Number of pages could not be extracted from PDF"
+
+
+class PDFtoPPMException(ConversionException):
+    error_code = 50
+    error_message = "Error converting PDF to Pixels (pdftoppm)"
+
+
+class PDFtoPPMInvalidHeader(PDFtoPPMException):
+    error_code = 51
+    error_message = "Error converting PDF to Pixels (Invalid PPM header)"
+
+
+class PDFtoPPMInvalidDepth(PDFtoPPMException):
+    error_code = 52
+    error_message = "Error converting PDF to Pixels (Invalid PPM depth)"
+
+
+def exception_from_error_code(error_code: int) -> Optional[ConversionException]:
+    """returns the conversion exception corresponding to the error code"""
+    for cls in ConversionException.get_subclasses():
+        if cls.error_code == error_code:
+            return cls()
+    raise ValueError(f"Unknown error code '{error_code}'")

dangerzone/isolation_provider/base.py

@@ -5,6 +5,7 @@ from typing import Callable, Optional
 
 from colorama import Fore, Style
 
+from ..conversion.errors import ConversionException
 from ..document import Document
 from ..util import replace_control_chars
 
@@ -36,7 +37,10 @@ class IsolationProvider(ABC):
         document.mark_as_converting()
         try:
             success = self._convert(document, ocr_lang)
-        except Exception:
+        except ConversionException as e:
+            success = False
+            self.print_progress_trusted(document, True, e.error_message, 0)
+        except Exception as e:
             success = False
             log.exception(
                 f"An exception occurred while converting document '{document.id}'"

dangerzone/isolation_provider/container.py

@@ -11,6 +11,7 @@ import sys
 import tempfile
 from typing import Any, Callable, List, Optional, Tuple
 
+from ..conversion.errors import exception_from_error_code
 from ..document import Document
 from ..util import (
     get_resource_path,
@@ -305,6 +306,9 @@ class Container(IsolationProvider):
 
         if ret != 0:
             log.error("documents-to-pixels failed")
+
+            # XXX Reconstruct exception from error code
+            raise exception_from_error_code(ret)  # type: ignore [misc]
         else:
             # TODO: validate convert to pixels output
 

dangerzone/isolation_provider/qubes.py

@@ -20,6 +20,7 @@ from .base import IsolationProvider
 log = logging.getLogger(__name__)
 
 from ..conversion.common import running_on_qubes
+from ..conversion.errors import exception_from_error_code
 from ..conversion.pixels_to_pdf import PixelsToPDF
 from ..util import get_resource_path, get_subprocess_startupinfo, get_tmp_dir
 from .base import MAX_CONVERSION_LOG_CHARS
@@ -38,6 +39,8 @@ def read_bytes(p: subprocess.Popen, buff_size: int) -> bytes:
 def read_int(p: subprocess.Popen) -> int:
     """Read 2 bytes from stdout, and decode them as int."""
     untrusted_int = p.stdout.read(2)  # type: ignore [union-attr]
+    if untrusted_int == b"":
+        raise ValueError("Nothing read from stdout (expected an integer)")
     return int.from_bytes(untrusted_int, signed=False)
 
 
@@ -104,7 +107,12 @@ class Qubes(IsolationProvider):
                     stderr=subprocess.DEVNULL,
                 )
 
-            n_pages = read_int(p)
+            try:
+                n_pages = read_int(p)
+            except ValueError:
+                error_code = p.wait()
+                # XXX Reconstruct exception from error code
+                raise exception_from_error_code(error_code)  # type: ignore [misc]
             if n_pages == 0:
                 # FIXME: Fail loudly in that case
                 return False