netfilter: xtables: check for standard verdicts in policies

This adds the second check that Rusty wanted to have a long time ago. :-)

Base chain policies must have absolute verdicts that cease processing
in the table, otherwise rule execution may continue in an unexpected
spurious fashion (e.g. next chain that follows in memory).

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

1 files changed, 19 insertions, 2 deletions

diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
index 064082d..7bc11ff 100644
--- a/net/ipv4/netfilter/arp_tables.c
+++ b/net/ipv4/netfilter/arp_tables.c
@@ -533,6 +533,21 @@ out:
 	return ret;
 }
 
+static bool check_underflow(struct arpt_entry *e)
+{
+	const struct arpt_entry_target *t;
+	unsigned int verdict;
+
+	if (!unconditional(&e->arp))
+		return false;
+	t = arpt_get_target(e);
+	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) != 0)
+		return false;
+	verdict = ((struct arpt_standard_target *)t)->verdict;
+	verdict = -verdict - 1;
+	return verdict == NF_DROP || verdict == NF_ACCEPT;
+}
+
 static inline int check_entry_size_and_hooks(struct arpt_entry *e,
 					     struct xt_table_info *newinfo,
 					     unsigned char *base,
@@ -564,8 +579,10 @@ static inline int check_entry_size_and_hooks(struct arpt_entry *e,
 		if ((unsigned char *)e - base == hook_entries[h])
 			newinfo->hook_entry[h] = hook_entries[h];
 		if ((unsigned char *)e - base == underflows[h]) {
-			if (!unconditional(&e->arp)) {
-				pr_err("Underflows must be unconditional\n");
+			if (!check_underflow(e)) {
+				pr_err("Underflows must be unconditional and "
+				       "use the STANDARD target with "
+				       "ACCEPT/DROP\n");
 				return -EINVAL;
 			}
 			newinfo->underflow[h] = underflows[h];