Attachment 132814 Details for Bug 101858 – [master] config-loader-expat: Tell Expat not to defend against hash collisions

[patch] [master] config-loader-expat: Tell Expat not to defend against hash collisions

master-config-loader-expat-Tell-Expat-not-to-defend-against.patch (text/plain), 2.64 KB, created by Simon McVittie on 2017-07-21 10:08:00 UTC

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Creator: Simon McVittie

Created: 2017-07-21 10:08:00 UTC

Size: 2.64 KB

patch

obsolete

>From e5909e6a28a6e416fa07e47bf7193e020d9cccdf Mon Sep 17 00:00:00 2001
>From: Simon McVittie <smcv@debian.org>
>Date: Fri, 21 Jul 2017 10:46:39 +0100
>Subject: [PATCH] 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
>Signed-off-by: Simon McVittie <smcv@debian.org>
>---
> bus/config-loader-expat.c | 14 ++++++++++++++
> configure.ac              |  8 ++++++++
> 2 files changed, 22 insertions(+)
>
>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);
>diff --git a/configure.ac b/configure.ac
>index 53c8b348..60a4427e 100644
>--- a/configure.ac
>+++ b/configure.ac
>@@ -935,6 +935,14 @@ AC_SUBST(DBUS_PATH_OR_ABSTRACT)
> 
> PKG_CHECK_MODULES([EXPAT], [expat])
> 
>+save_cflags="$CFLAGS"
>+save_libs="$LIBS"
>+CFLAGS="$CFLAGS $EXPAT_CFLAGS"
>+LIBS="$LIBS $EXPAT_LIBS"
>+AC_CHECK_FUNCS([XML_SetHashSalt])
>+CFLAGS="$save_cflags"
>+LIBS="$save_libs"
>+
> # Thread lib detection
> AC_ARG_VAR([THREAD_LIBS])
> save_libs="$LIBS"
>-- 
>2.13.3
>

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