Mask some extra paths in gVisor's OCI config

Mask some paths of the outer container in the OCI config of the inner
container. This is done to avoid leaking any sensitive information from
Podman / Docker / gVisor, since we reuse the same rootfs

Refs #1048

dangerzone/container_helpers/entrypoint.py

@@ -59,17 +59,82 @@ def log(message: str, *values: typing.Any) -> None:
     "root": {"path": "/", "readonly": True},
     "hostname": "dangerzone",
     "mounts": [
+        # Mask almost every system directory of the outer container, by mounting tmpfs
+        # on top of them. This is done to avoid leaking any sensitive information,
+        # either mounted by Podman/Docker, or when gVisor runs, since we reuse the same
+        # rootfs. We basically mask everything except for `/usr`, `/bin`, `/lib`,
+        # and `/etc`.
+        #
+        # Note that we set `--root /home/dangerzone/.containers` for the directory where
+        # gVisor will create files at runtime, which means that in principle, we are
+        # covered by the masking of `/home/dangerzone` that follows below.
+        #
+        # Finally, note that the following list has been taken from the dirs in our
+        # container image, and double-checked against the top-level dirs listed in the
+        # Filesystem Hierarchy Standard (FHS) [1]. It would be nice to have an allowlist
+        # approach instead of a denylist, but FHS is such an old standard that we don't
+        # expect any new top-level dirs to pop up any time soon.
+        #
+        # [1] https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
+        {
+            "destination": "/boot",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
+        {
+            "destination": "/dev",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev"],
+        },
+        {
+            "destination": "/home",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
+        {
+            "destination": "/media",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
+        {
+            "destination": "/mnt",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
         {
             "destination": "/proc",
             "type": "proc",
             "source": "proc",
         },
         {
-            "destination": "/dev",
+            "destination": "/root",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
+        {
+            "destination": "/run",
             "type": "tmpfs",
             "source": "tmpfs",
             "options": ["nosuid", "noexec", "nodev"],
         },
+        {
+            "destination": "/sbin",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
+        {
+            "destination": "/srv",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev", "ro"],
+        },
         {
             "destination": "/sys",
             "type": "tmpfs",
@@ -82,6 +147,27 @@ def log(message: str, *values: typing.Any) -> None:
             "source": "tmpfs",
             "options": ["nosuid", "noexec", "nodev"],
         },
+        {
+            "destination": "/var",
+            "type": "tmpfs",
+            "source": "tmpfs",
+            "options": ["nosuid", "noexec", "nodev"],
+        },
+        # Also mask some files that are usually mounted by Docker / Podman. These files
+        # should not contain any sensitive information, since we use the `--network
+        # none` flag, but we want to make sure in any case.
+        {
+            "destination": "/etc/hostname",
+            "type": "bind",
+            "source": "/dev/null",
+            "options": ["rbind", "ro"],
+        },
+        {
+            "destination": "/etc/hosts",
+            "type": "bind",
+            "source": "/dev/null",
+            "options": ["rbind", "ro"],
+        },
         # LibreOffice needs a writable home directory, so just mount a tmpfs
         # over it.
         {