platform: don't accept lowering IPv6 hop-limit from RA (CVE-2015-2924)

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6fd99094de2b83d1d4c8457f2c83483b2828e75a
http://seclists.org/oss-sec/2015/q2/46
https://bugzilla.redhat.com/show_bug.cgi?id=1209902
https://bugzilla.redhat.com/show_bug.cgi?id=1209903

4 files changed, 38 insertions, 16 deletions

diff --git a/src/devices/nm-device.c b/src/devices/nm-device.c
index 1de4e4730b..fb892be3db 100644
--- a/src/devices/nm-device.c
+++ b/src/devices/nm-device.c
@@ -4303,14 +4303,8 @@ rdisc_config_changed (NMRDisc *rdisc, NMRDiscConfigMap changed, NMDevice *self)
 		}
 	}
 
-	/* hop_limit == 0 is a special value "unspecified", so do not touch
-	 * in this case */
-	if (changed & NM_RDISC_CONFIG_HOP_LIMIT && rdisc->hop_limit > 0) {
-		char val[16];
-
-		g_snprintf (val, sizeof (val), "%d", rdisc->hop_limit);
-		nm_device_ipv6_sysctl_set (self, "hop_limit", val);
-	}
+	if (changed & NM_RDISC_CONFIG_HOP_LIMIT)
+		nm_platform_sysctl_set_ip6_hop_limit_safe (NM_PLATFORM_GET, nm_device_get_ip_iface (self), rdisc->hop_limit);
 
 	if (changed & NM_RDISC_CONFIG_MTU)
 		priv->ip6_mtu = rdisc->mtu;
diff --git a/src/nm-iface-helper.c b/src/nm-iface-helper.c
index 94a6f0550b..979c2ba665 100644
--- a/src/nm-iface-helper.c
+++ b/src/nm-iface-helper.c
@@ -226,14 +226,8 @@ rdisc_config_changed (NMRDisc *rdisc, NMRDiscConfigMap changed, gpointer user_da
 		/* Unsupported until systemd DHCPv6 is ready */
 	}
 
-	/* hop_limit == 0 is a special value "unspecified", so do not touch
-	 * in this case */
-	if (changed & NM_RDISC_CONFIG_HOP_LIMIT && rdisc->hop_limit > 0) {
-		char val[16];
-
-		g_snprintf (val, sizeof (val), "%d", rdisc->hop_limit);
-		nm_platform_sysctl_set (NM_PLATFORM_GET, nm_utils_ip6_property_path (global_opt.ifname, "hop_limit"), val);
-	}
+	if (changed & NM_RDISC_CONFIG_HOP_LIMIT)
+		nm_platform_sysctl_set_ip6_hop_limit_safe (NM_PLATFORM_GET, global_opt.ifname, rdisc->hop_limit);
 
 	if (changed & NM_RDISC_CONFIG_MTU) {
 		char val[16];
diff --git a/src/platform/nm-platform.c b/src/platform/nm-platform.c
index 1a7b9bc182..518b69dd02 100644
--- a/src/platform/nm-platform.c
+++ b/src/platform/nm-platform.c
@@ -271,6 +271,38 @@ nm_platform_sysctl_set (NMPlatform *self, const char *path, const char *value)
 	return klass->sysctl_set (self, path, value);
 }
 
+gboolean
+nm_platform_sysctl_set_ip6_hop_limit_safe (NMPlatform *self, const char *iface, int value)
+{
+	const char *path;
+	gint64 cur;
+
+	/* the hop-limit provided via RA is uint8. */
+	if (value > 0xFF)
+		return FALSE;
+
+	/* don't allow unreasonable small values */
+	if (value < 10)
+		return FALSE;
+
+	path = nm_utils_ip6_property_path (iface, "hop_limit");
+	cur = nm_platform_sysctl_get_int_checked (self, path, 10, 1, G_MAXINT32, -1);
+
+	/* only allow increasing the hop-limit to avoid DOS by an attacker
+	 * setting a low hop-limit (CVE-2015-2924, rh#1209902) */
+
+	if (value < cur)
+		return FALSE;
+	if (value != cur) {
+		char svalue[20];
+
+		sprintf (svalue, "%d", value);
+		nm_platform_sysctl_set (self, path, svalue);
+	}
+
+	return TRUE;
+}
+
 /**
  * nm_platform_sysctl_get:
  * @self: platform instance
diff --git a/src/platform/nm-platform.h b/src/platform/nm-platform.h
index 5cd5ae8e0c..67a9639cda 100644
--- a/src/platform/nm-platform.h
+++ b/src/platform/nm-platform.h
@@ -508,6 +508,8 @@ char *nm_platform_sysctl_get (NMPlatform *self, const char *path);
 gint32 nm_platform_sysctl_get_int32 (NMPlatform *self, const char *path, gint32 fallback);
 gint64 nm_platform_sysctl_get_int_checked (NMPlatform *self, const char *path, guint base, gint64 min, gint64 max, gint64 fallback);
 
+gboolean nm_platform_sysctl_set_ip6_hop_limit_safe (NMPlatform *self, const char *iface, int value);
+
 gboolean nm_platform_link_get (NMPlatform *self, int ifindex, NMPlatformLink *link);
 GArray *nm_platform_link_get_all (NMPlatform *self);
 gboolean nm_platform_dummy_add (NMPlatform *self, const char *name);