netfilter: x_tables: fix compat match/target pad out-of-bound write

xt_compat_match/target_from_user doesn't check that zeroing the area
to start of next rule won't write past end of allocated ruleset blob.

Remove this code and zero the entire blob beforehand.

Reported-by: syzbot+cfc0247ac173f597aaaa@syzkaller.appspotmail.com
Reported-by: Andy Nguyen <theflow@google.com>
Fixes: 9fa492cd ("[NETFILTER]: x_tables: simplify compat API")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/arp_tables.c

@@ -1193,6 +1193,8 @@ static int translate_compat_table(struct net *net,
 	if (!newinfo)
 		goto out_unlock;
 
+	memset(newinfo->entries, 0, size);
+
 	newinfo->number = compatr->num_entries;
 	for (i = 0; i < NF_ARP_NUMHOOKS; i++) {
 		newinfo->hook_entry[i] = compatr->hook_entry[i];

net/ipv4/netfilter/ip_tables.c

@@ -1428,6 +1428,8 @@ translate_compat_table(struct net *net,
 	if (!newinfo)
 		goto out_unlock;
 
+	memset(newinfo->entries, 0, size);
+
 	newinfo->number = compatr->num_entries;
 	for (i = 0; i < NF_INET_NUMHOOKS; i++) {
 		newinfo->hook_entry[i] = compatr->hook_entry[i];

net/ipv6/netfilter/ip6_tables.c

@@ -1443,6 +1443,8 @@ translate_compat_table(struct net *net,
 	if (!newinfo)
 		goto out_unlock;
 
+	memset(newinfo->entries, 0, size);
+
 	newinfo->number = compatr->num_entries;
 	for (i = 0; i < NF_INET_NUMHOOKS; i++) {
 		newinfo->hook_entry[i] = compatr->hook_entry[i];

net/netfilter/x_tables.c

@@ -733,7 +733,7 @@ void xt_compat_match_from_user(struct xt_entry_match *m, void **dstptr,
 {
 	const struct xt_match *match = m->u.kernel.match;
 	struct compat_xt_entry_match *cm = (struct compat_xt_entry_match *)m;
-	int pad, off = xt_compat_match_offset(match);
+	int off = xt_compat_match_offset(match);
 	u_int16_t msize = cm->u.user.match_size;
 	char name[sizeof(m->u.user.name)];
 
@@ -743,9 +743,6 @@ void xt_compat_match_from_user(struct xt_entry_match *m, void **dstptr,
 		match->compat_from_user(m->data, cm->data);
 	else
 		memcpy(m->data, cm->data, msize - sizeof(*cm));
-	pad = XT_ALIGN(match->matchsize) - match->matchsize;
-	if (pad > 0)
-		memset(m->data + match->matchsize, 0, pad);
 
 	msize += off;
 	m->u.user.match_size = msize;
@@ -1116,7 +1113,7 @@ void xt_compat_target_from_user(struct xt_entry_target *t, void **dstptr,
 {
 	const struct xt_target *target = t->u.kernel.target;
 	struct compat_xt_entry_target *ct = (struct compat_xt_entry_target *)t;
-	int pad, off = xt_compat_target_offset(target);
+	int off = xt_compat_target_offset(target);
 	u_int16_t tsize = ct->u.user.target_size;
 	char name[sizeof(t->u.user.name)];
 
@@ -1126,9 +1123,6 @@ void xt_compat_target_from_user(struct xt_entry_target *t, void **dstptr,
 		target->compat_from_user(t->data, ct->data);
 	else
 		memcpy(t->data, ct->data, tsize - sizeof(*ct));
-	pad = XT_ALIGN(target->targetsize) - target->targetsize;
-	if (pad > 0)
-		memset(t->data + target->targetsize, 0, pad);
 
 	tsize += off;
 	t->u.user.target_size = tsize;