netfilter: x_tables: check standard verdicts in core

Userspace must provide a valid verdict to the standard target.

The verdict can be either a jump (signed int > 0), or a return code.

Allowed return codes are either RETURN (pop from stack), NF_ACCEPT, DROP
and QUEUE (latter is allowed for legacy reasons).

Jump offsets (verdict > 0) are checked in more detail later on when
loop-detection is performed.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/arp_tables.c

@@ -334,11 +334,6 @@ static int mark_source_chains(const struct xt_table_info *newinfo,
 			     t->verdict < 0) || visited) {
 				unsigned int oldpos, size;
 
-				if ((strcmp(t->target.u.user.name,
-					    XT_STANDARD_TARGET) == 0) &&
-				    t->verdict < -NF_MAX_VERDICT - 1)
-					return 0;
-
 				/* Return: backtrack through the last
 				 * big jump.
 				 */

net/ipv4/netfilter/ip_tables.c

@@ -402,11 +402,6 @@ mark_source_chains(const struct xt_table_info *newinfo,
 			     t->verdict < 0) || visited) {
 				unsigned int oldpos, size;
 
-				if ((strcmp(t->target.u.user.name,
-					    XT_STANDARD_TARGET) == 0) &&
-				    t->verdict < -NF_MAX_VERDICT - 1)
-					return 0;
-
 				/* Return: backtrack through the last
 				   big jump. */
 				do {

net/ipv6/netfilter/ip6_tables.c

@@ -420,11 +420,6 @@ mark_source_chains(const struct xt_table_info *newinfo,
 			     t->verdict < 0) || visited) {
 				unsigned int oldpos, size;
 
-				if ((strcmp(t->target.u.user.name,
-					    XT_STANDARD_TARGET) == 0) &&
-				    t->verdict < -NF_MAX_VERDICT - 1)
-					return 0;
-
 				/* Return: backtrack through the last
 				   big jump. */
 				do {

net/netfilter/x_tables.c

@@ -654,6 +654,31 @@ struct compat_xt_standard_target {
 	compat_uint_t verdict;
 };
 
+static bool verdict_ok(int verdict)
+{
+	if (verdict > 0)
+		return true;
+
+	if (verdict < 0) {
+		int v = -verdict - 1;
+
+		if (verdict == XT_RETURN)
+			return true;
+
+		switch (v) {
+		case NF_ACCEPT: return true;
+		case NF_DROP: return true;
+		case NF_QUEUE: return true;
+		default:
+			break;
+		}
+
+		return false;
+	}
+
+	return false;
+}
+
 int xt_compat_check_entry_offsets(const void *base, const char *elems,
 				  unsigned int target_offset,
 				  unsigned int next_offset)
@@ -675,9 +700,15 @@ int xt_compat_check_entry_offsets(const void *base, const char *elems,
 	if (target_offset + t->u.target_size > next_offset)
 		return -EINVAL;
 
-	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0 &&
-	    COMPAT_XT_ALIGN(target_offset + sizeof(struct compat_xt_standard_target)) != next_offset)
-		return -EINVAL;
+	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0) {
+		const struct compat_xt_standard_target *st = (const void *)t;
+
+		if (COMPAT_XT_ALIGN(target_offset + sizeof(*st)) != next_offset)
+			return -EINVAL;
+
+		if (!verdict_ok(st->verdict))
+			return -EINVAL;
+	}
 
 	/* compat_xt_entry match has less strict alignment requirements,
 	 * otherwise they are identical.  In case of padding differences
@@ -757,9 +788,15 @@ int xt_check_entry_offsets(const void *base,
 	if (target_offset + t->u.target_size > next_offset)
 		return -EINVAL;
 
-	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0 &&
-	    XT_ALIGN(target_offset + sizeof(struct xt_standard_target)) != next_offset)
-		return -EINVAL;
+	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0) {
+		const struct xt_standard_target *st = (const void *)t;
+
+		if (XT_ALIGN(target_offset + sizeof(*st)) != next_offset)
+			return -EINVAL;
+
+		if (!verdict_ok(st->verdict))
+			return -EINVAL;
+	}
 
 	return xt_check_entry_match(elems, base + target_offset,
 				    __alignof__(struct xt_entry_match));