config-loader-expat: Tell Expat not to defend against hash collisions

By default, Expat uses cryptographic-quality random numbers as a salt for
its hash algorithm, and since 2.2.1 it gets them from the getrandom
syscall on Linux. That syscall refuses to return any entropy until the
kernel's CSPRNG (random pool) has been initialized. Unfortunately, this
can take as long as 40 seconds on embedded devices with few entropy
sources, which is too long: if the system dbus-daemon blocks for that
length of time, important D-Bus clients like systemd and systemd-logind
time out and fail to connect to it.

We're parsing small configuration files here, and we trust them
completely, so we don't need to defend against hash collisions: nobody
is going to be crafting them to cause pathological performance.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101858
Tested-by: Christopher Hewitt <hewitt@ieee.org>
[smcv: Adjust build-system changes for 1.11.x]
Signed-off-by: Simon McVittie <smcv@debian.org>
Reviewed-by: Philip Withnall <withnall@endlessm.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/bus/config-loader-expat.c b/bus/config-loader-expat.c
index f59942e3..6cbd37eb 100644
--- a/bus/config-loader-expat.c
+++ b/bus/config-loader-expat.c
@@ -203,6 +203,20 @@ bus_config_load (const DBusString      *file,
       goto failed;
     }
 
+  /* We do not need protection against hash collisions (CVE-2012-0876)
+   * because we are only parsing trusted XML; and if we let Expat block
+   * waiting for the CSPRNG to be initialized, as it does by default to
+   * defeat CVE-2012-0876, it can cause timeouts during early boot on
+   * entropy-starved embedded devices.
+   *
+   * TODO: When Expat gets a more explicit API for this than
+   * XML_SetHashSalt, check for that too, and use it preferentially.
+   * https://github.com/libexpat/libexpat/issues/91 */
+#if defined(HAVE_XML_SETHASHSALT)
+  /* Any nonzero number will do. https://xkcd.com/221/ */
+  XML_SetHashSalt (expat, 4);
+#endif
+
   if (!_dbus_string_get_dirname (file, &dirname))
     {
       dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);