Added option for specifying a user to run external extraction utiltie…

…s as

src/binwalk/modules/extractor.py

@@ -92,6 +92,11 @@ class Extractor(Module):
                type=int,
                kwargs={'max_count': 0},
                description='Limit the number of extracted files'),
+        Option(short='0',
+               long='run-as',
+               type=str,
+               kwargs={'runas_user': 0},
+               description="Execute external extraction utilities with the specified user's privileges"),
         #Option(short='u',
         #       long='limit',
         #       type=int,
@@ -109,10 +114,6 @@ class Extractor(Module):
                long='subdirs',
                kwargs={'extract_into_subdirs': True},
                description="Extract into sub-directories named by the offset"),
-        Option(short='0',
-               long='unsafe',
-               kwargs={'unsafe_extraction': True},
-               description="Execute external extractions without dropping privileges"),
     ]
 
     KWARGS = [
@@ -127,25 +128,24 @@ class Extractor(Module):
         Kwarg(name='manual_rules', default=[]),
         Kwarg(name='matryoshka', default=0),
         Kwarg(name='enabled', default=False),
-        Kwarg(name='unsafe_extraction', default=False),
+        Kwarg(name='runas_user', default=None),
     ]
 
     def load(self):
-        self.unpriv_uid = None
-        self.unpriv_gid = None
-
-        if self.enabled is True and self.unsafe_extraction is False:
-            if os.getuid() != 0:
-                raise ModuleException("Binwalk requires root privileges to safely execute external extraction utilities as an unprivileged user.")
-
-            self.shell_call('useradd %s' % self.UNPRIVILEGED_USER_NAME)
-
-            user_info = pwd.getpwnam(self.UNPRIVILEGED_USER_NAME)
-            self.unpriv_uid = user_info.pw_uid
-            self.unpriv_gid = user_info.pw_gid
-        else:
-            self.unpriv_uid = os.getuid()
-            self.unpriv_gid = os.getgid()
+        self.runas_uid = None
+        self.runas_gid = None
+
+        if self.enabled is True:
+            if self.runas_user is None:
+                if os.getuid() == 0:
+                    raise ModuleException("Binwalk extraction uses many third party utilities, which may not be secure.\nIf you wish to have extraction utilities executed as the current user, use --run-as=<user name>.")
+
+                self.runas_uid = os.getuid()
+                self.runas_gid = os.getgid()
+            else:
+                user_info = pwd.getpwnam(self.runas_user)
+                self.runas_uid = user_info.pw_uid
+                self.runas_gid = user_info.pw_gid
 
         # Holds a list of extraction rules loaded either from a file or when
         # manually specified.
@@ -560,8 +560,8 @@ def build_output_directory(self, path):
         else:
             output_directory = self.extraction_directories[path]
 
-        # Make sure unpriv user can access this directory
-        os.chown(output_directory, self.unpriv_uid, self.unpriv_gid)
+        # Make sure runas user can access this directory
+        os.chown(output_directory, self.runas_uid, self.runas_gid)
 
         return output_directory
 
@@ -856,8 +856,8 @@ def _dd(self, file_name, offset, size, extension, output_file_name=None):
             fdout.close()
             fdin.close()
 
-            # Make sure unprivileged user can access this file
-            os.chown(fname, self.unpriv_uid, self.unpriv_gid)
+            # Make sure runasileged user can access this file
+            os.chown(fname, self.runas_uid, self.runas_gid)
         except KeyboardInterrupt as e:
             raise e
         except Exception as e:
@@ -938,30 +938,24 @@ def execute(self, cmd, fname, codes=[0, None]):
         return (retval, '&&'.join(command_list))
 
     def shell_call(self, command):
-        # Fork a child to drop privs
         child_pid = os.fork()
         if child_pid is 0:
-            # Switch to unprivileged user, if one has been set
-            if self.unpriv_uid is not None and self.unpriv_gid is not None:
-                binwalk.core.common.debug("Dropping privileges to %d:%d" % (self.unpriv_uid, self.unpriv_gid))
-                os.setgid(self.unpriv_uid)
-                os.setuid(self.unpriv_gid)
-
-            # If not in debug mode, create a temporary file to redirect
-            # stdout and stderr to
+            # Switch to the run-as user privileges, if one has been set
+            if self.runas_uid is not None and self.runas_gid is not None:
+                binwalk.core.common.debug("Switching privileges to %s (%d:%d)" % (self.runas_user, self.runas_uid, self.runas_gid))
+                os.setgid(self.runas_uid)
+                os.setuid(self.runas_gid)
+
+            # If not in debug mode, redirect output to /dev/null
             if not binwalk.core.common.DEBUG:
-                tmp = tempfile.TemporaryFile()
+                tmp = subprocess.DEVNULL
             else:
                 tmp = None
 
             # Execute command
             binwalk.core.common.debug("subprocess.call(%s, stdout=%s, stderr=%s)" % (command, str(tmp), str(tmp)))
             rval = subprocess.call(shlex.split(command), stdout=tmp, stderr=tmp)
 
-            # Clean up temp file
-            if tmp is not None:
-                tmp.close()
-
             sys.exit(rval)
         else:
             return os.wait()[1]