labels: don't crash on NULL labelmap

When CONNLABEL_CFG isn't available (/etc/xtables/connlabel.conf),
conntrack tool crashes:

[marcos@Icarus ~]$ conntrack -l something
nfct_labelmap_new: No such file or directory
Segmentation fault (core dumped)

I can see this problem in Fedora 26, because connlabel.conf does not
come along the conntrack/libnetfilter packages. This problem happens because
conntrack calls nfct_labelmap_new, which resides on
libnetfilter_conntrack. So this lib returns NULL because CONNLABEL_CFG
is not present, and then NULL is assigned to the global var called
labelmap on conntrack. Later, get_label is called, passing NULL to the
library, and __label_get_bit is called and deferences labelmap without
check, which leads to a crash.

With this patch the crash does not happen anymore,
and an error message is displayed:

conntrack -l something
nfct_labelmap_new: No such file or directory
conntrack v1.4.4 (conntrack-tools): unknown label 'something'

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/conntrack/labels.c b/src/conntrack/labels.c
index 8048076..ef85b6e 100644
--- a/src/conntrack/labels.c
+++ b/src/conntrack/labels.c
@@ -51,8 +51,14 @@ static unsigned int hash_name(const char *name)
 
 int __labelmap_get_bit(struct nfct_labelmap *m, const char *name)
 {
-	unsigned int i = hash_name(name);
-	struct labelmap_bucket *list = m->map_name[i];
+	struct labelmap_bucket *list;
+	unsigned int i;
+
+	if (!m)
+		return -1;
+
+	i = hash_name(name);
+	list = m->map_name[i];
 
 	while (list) {
 		if (strcmp(name, list->name) == 0)