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[libvirt] [PATCH] network: don't require private addresses if dnsmasq uses SO_BINDTODEVICE

This is yet another refinement to the fix for CVE-2012-3411:


It turns out that it would be very intrusive to correctly backport the
entire --bind-dynamic option to older dnsmasq versions
(e.g. dnsmasq-2.48 that is used on RHEL6.x and CentOS 6.x), but very
simple to patch those versions to just use SO_BINDTODEVICE on all
their listening sockets (SO_BINDTODEVICE also has the desired effect
of permitting only traffic that was received on the interface(s) where
dnsmasq was set to listen.)

This patch modifies the dnsmasq capabilities detection to detect the

    --bind-interfaces with SO_BINDTODEVICE

in the output of "dnsmasq --version", and in that case realize that
using the old --bind-interfaces option is just as safe as
--bind-dynamic (and therefore *not* forbid creation of networks that
use public IP address ranges).

If -bind-dynamic is available, it is still preferred over

Note that this patch does no harm in upstream, or in any distro's
downstream if it happens to end up there, but builds for distros that
have a new enough dnsmasq to support --bind-dynamic do *NOT* need to
specifically backport this patch; it's only required for distro
releases that have dnsmasq too old to have --bind-dynamic (and those
distros will need to add the SO_BINDTODEVICE patch to dnsmasq,
*including the extra string in the --version output*, as well.
 src/network/bridge_driver.c |  6 ++++--
 src/util/dnsmasq.c          | 15 ++++++++++++---
 src/util/dnsmasq.h          |  1 +
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
index c62e729..fdd9382 100644
--- a/src/network/bridge_driver.c
+++ b/src/network/bridge_driver.c
@@ -725,13 +725,15 @@ networkDnsmasqConfContents(virNetworkObjPtr network,
              * dnsmasq doesn't have bind-dynamic, only allow listening on
              * private/local IP addresses (see RFC1918/RFC3484/RFC4193)
-            if (!virSocketAddrIsPrivate(&tmpipdef->address)) {
+            if (!dnsmasqCapsGet(caps, DNSMASQ_CAPS_BINDTODEVICE) &&
+                !virSocketAddrIsPrivate(&tmpipdef->address)) {
                 unsigned long version = dnsmasqCapsGetVersion(caps);
                                _("Publicly routable address %s is prohibited. "
                                  "The version of dnsmasq on this host (%d.%d) doesn't "
-                                 "support the bind-dynamic option, which is required "
+                                 "support the bind-dynamic option or use SO_BINDTODEVICE "
+                                 "on listening sockets, one of which is required "
                                  "for safe operation on a publicly routable subnet "
                                  "(see CVE-2012-3411). You must either upgrade dnsmasq, "
                                  "or use a private/local subnet range for this network "
diff --git a/src/util/dnsmasq.c b/src/util/dnsmasq.c
index e8eab1e..ba51447 100644
--- a/src/util/dnsmasq.c
+++ b/src/util/dnsmasq.c
@@ -664,10 +664,19 @@ dnsmasqCapsSetFromBuffer(dnsmasqCapsPtr caps, const char *buf)
     if (strstr(buf, "--bind-dynamic"))
         dnsmasqCapsSet(caps, DNSMASQ_CAPS_BIND_DYNAMIC);
-    VIR_INFO("dnsmasq version is %d.%d, --bind-dynamic is %s",
+    /* if this string is a part of the --version output, dnsmasq
+     * has been patched to use SO_BINDTODEVICE when listening,
+     * so that it will only accept requests that arrived on the
+     * listening interface(s)
+     */
+    if (strstr(buf, "--bind-interfaces with SO_BINDTODEVICE"))
+        dnsmasqCapsSet(caps, DNSMASQ_CAPS_BINDTODEVICE);
+    VIR_INFO("dnsmasq version is %d.%d, --bind-dynamic is %spresent, "
+             "SO_BINDTODEVICE is %sin use",
              (int)caps->version / 1000000, (int)(caps->version % 1000000) / 1000,
-             dnsmasqCapsGet(caps, DNSMASQ_CAPS_BIND_DYNAMIC)
-             ? "present" : "NOT present");
+             dnsmasqCapsGet(caps, DNSMASQ_CAPS_BIND_DYNAMIC) ? "" : "NOT ",
+             dnsmasqCapsGet(caps, DNSMASQ_CAPS_BIND_DYNAMIC) ? "" : "NOT ");
     return 0;
diff --git a/src/util/dnsmasq.h b/src/util/dnsmasq.h
index 7a39232..b83bc96 100644
--- a/src/util/dnsmasq.h
+++ b/src/util/dnsmasq.h
@@ -68,6 +68,7 @@ typedef struct
 typedef enum {
    DNSMASQ_CAPS_BIND_DYNAMIC = 0, /* support for --bind-dynamic */
+   DNSMASQ_CAPS_BINDTODEVICE = 1, /* uses SO_BINDTODEVICE for --bind-interfaces */
    DNSMASQ_CAPS_LAST,             /* this must always be the last item */
 } dnsmasqCapsFlags;

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