CVE-2025-68206: netfilter: nft_ct: add seqadj extension for natted connections

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nftct: add seqadj extension for natted connections

Sequence adjustment may be required for FTP traffic with PASV/EPSV modes. due to need to re-write packet payload (IP, port) on the ftp control connection. This can require changes to the TCP length and expected seq / ackseq.

The easiest way to reproduce this issue is with PASV mode. Example ruleset: table inet ftpnat { ct helper ftphelper { type "ftp" protocol tcp l3proto inet }

chain prerouting { type filter hook prerouting priority 0; policy accept; tcp dport 21 ct state new ct helper set "ftphelper" } } table ip nat { chain prerouting { type nat hook prerouting priority -100; policy accept; tcp dport 21 dnat ip prefix to ip daddr map { 192.168.100.1 : 192.168.13.2/32 } }

chain postrouting { type nat hook postrouting priority 100 ; policy accept; tcp sport 21 snat ip prefix to ip saddr map { 192.168.13.2 : 192.168.100.1/32 } } }

Note that the ftp helper gets assigned after the dnat setup.

The inverse (nat after helper assign) is handled by an existing check in nfnatsetupinfo() and will not show the problem.

Topoloy:

+-------------------+ +----------------------------------+ | FTP: 192.168.13.2 | <-> | NAT: 192.168.13.3, 192.168.100.1 | +-------------------+ +----------------------------------+ | +-----------------------+ | Client: 192.168.100.2 | +-----------------------+

ftp nat changes do not work as expected in this case: Connected to 192.168.100.1. [..] ftp> epsv EPSV/EPRT on IPv4 off. ftp> ls 227 Entering passive mode (192,168,100,1,209,129). 421 Service not available, remote server has closed connection.

Kernel logs: Missing nfctseqadjextadd() setup call WARNING: CPU: 1 PID: 0 at net/netfilter/nfconntrackseqadj.c:41 [..] nfnatmangletcppacket+0x100/0x160 [nfnat] nfnatftp+0x142/0x280 [nfnatftp] help+0x4d1/0x880 [nfconntrackftp] nfconfirm+0x122/0x2e0 [nfconntrack] nfhookslow+0x3c/0xb0 ..

Fix this by adding the required extension when a conntrack helper is assigned to a connection that has a nat binding.