netfilter: x_tables: xt_mttg_seq_next should increase position index

stable inclusion
from linux-4.19.111
commit 80a12a6f03577de7077a04705b2eeb8078653816

--------------------------------

commit ee84f19c upstream.

If .next function does not change position index,
following .show function will repeat output related
to current position index.

Without patch:
 # dd if=/proc/net/ip_tables_matches  # original file output
 conntrack
 conntrack
 conntrack
 recent
 recent
 icmp
 udplite
 udp
 tcp
 0+1 records in
 0+1 records out
 65 bytes copied, 5.4074e-05 s, 1.2 MB/s

 # dd if=/proc/net/ip_tables_matches bs=62 skip=1
 dd: /proc/net/ip_tables_matches: cannot skip to specified offset
 cp   <<< end of  last line
 tcp  <<< and then unexpected whole last line once again
 0+1 records in
 0+1 records out
 7 bytes copied, 0.000102447 s, 68.3 kB/s

Cc: stable@vger.kernel.org
Fixes: 1f4aace6 ("fs/seq_file.c: simplify seq_file iteration code ...")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206283Signed-off-by: NVasily Averin <vvs@virtuozzo.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NLi Aichun <liaichun@huawei.com>
Reviewed-by: Nguodeqing <geffrey.guo@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/netfilter/x_tables.c

@@ -1556,6 +1556,9 @@ static void *xt_mttg_seq_next(struct seq_file *seq, void *v, loff_t *ppos,
 	uint8_t nfproto = (unsigned long)PDE_DATA(file_inode(seq->file));
 	struct nf_mttg_trav *trav = seq->private;
 
+	if (ppos != NULL)
+		++(*ppos);
+
 	switch (trav->class) {
 	case MTTG_TRAV_INIT:
 		trav->class = MTTG_TRAV_NFP_UNSPEC;
@@ -1581,9 +1584,6 @@ static void *xt_mttg_seq_next(struct seq_file *seq, void *v, loff_t *ppos,
 	default:
 		return NULL;
 	}
-
-	if (ppos != NULL)
-		++*ppos;
 	return trav;
 }