In older versions of bubblewrap, the --dir argument could only create
directories with mode 0755 (world-readable). The default mode is
still 0755, but since bubblewrap 0.5.0, we can change this to the mode
recommended by the basedirs spec (0700, private to user).
Since version 1.11.1, the --dir used here is normally redundant, because
each app-ID gets its own XDG_RUNTIME_DIR imported from the host
(it's actually $XDG_RUNTIME_DIR/.flatpak/$FLATPAK_ID/xdg-run on the host)
and those are correctly created with mode 0700. The one case where this
change makes a difference is that a sub-sandbox, created by
flatpak-spawn --sandbox or equivalent, does not share the common
XDG_RUNTIME_DIR and instead received an XDG_RUNTIME_DIR private to that
instance, with its permissions coming from this --dir argument.
This is not a security issue, because processes outside the sandbox can
only access this directory via /proc/$pid/root, which is only accessible
by processes that would be allowed to trace the sandboxed process
(either the same uid or a privileged process), resulting in behaviour
similar to the canonical 0700 mode. Inside the sandbox, all processes
have the same uid, so 0700, 0755 and even 0777 permissions would be
essentially equivalent.
However, some libraries that interact with XDG_RUNTIME_DIR, most notably
Qt, verify that its mode is 0700 as a hardening mechanism, and these
libraries did not work inside a Flatpak sandbox without being patched
to be more permissive.
Resolves: https://github.com/flatpak/flatpak/issues/3397
Signed-off-by: Simon McVittie <smcv@collabora.com>
7c40ba3b91