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netfilter: Exclude LEGACY TABLES on PREEMPT_RT.

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The seqcount xt_recseq is used to synchronize the replacement of
xt_table::private in xt_replace_table() against all readers such as
ipt_do_table()

To ensure that there is only one writer, the writing side disables
bottom halves. The sequence counter can be acquired recursively. Only the
first invocation modifies the sequence counter (signaling that a writer
is in progress) while the following (recursive) writer does not modify
the counter.
The lack of a proper locking mechanism for the sequence counter can lead
to live lock on PREEMPT_RT if the high prior reader preempts the
writer. Additionally if the per-CPU lock on PREEMPT_RT is removed from
local_bh_disable() then there is no synchronisation for the per-CPU
sequence counter.

The affected code is "just" the legacy netfilter code which is replaced
by "netfilter tables". That code can be disabled without sacrificing
functionality because everything is provided by the newer
implementation. This will only requires the usage of the "-nft" tools
instead of the "-legacy" ones.
The long term plan is to remove the legacy code so lets accelerate the
progress.

Relax dependencies on iptables legacy, replace select with depends on,
this should cause no harm to existing kernel configs and users can still
toggle IP{6}_NF_IPTABLES_LEGACY in any case.
Make EBTABLES_LEGACY, IPTABLES_LEGACY and ARPTABLES depend on
NETFILTER_XTABLES_LEGACY. Hide xt_recseq and its users,
xt_register_table() and xt_percpu_counter_alloc() behind
NETFILTER_XTABLES_LEGACY. Let NETFILTER_XTABLES_LEGACY depend on
!PREEMPT_RT.

This will break selftest expecing the legacy options enabled and will be
addressed in a following patch.

Co-developed-by: Florian Westphal <fw@strlen.de>
Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Pablo Neira Ayuso 2025-06-30 17:44:23 +02:00
parent 29f0f4cefc
commit 9fce66583f
5 changed files with 47 additions and 32 deletions
Download patch file Download diff file Expand all files Collapse all files

10
net/bridge/netfilter/Kconfig
View file

@ -42,8 +42,8 @@ config NF_CONNTRACK_BRIDGE
# old sockopt interface and eval loop
config BRIDGE_NF_EBTABLES_LEGACY
tristate "Legacy EBTABLES support"
depends on BRIDGE && NETFILTER_XTABLES
default n
depends on BRIDGE && NETFILTER_XTABLES_LEGACY
default n
help
Legacy ebtables packet/frame classifier.
This is not needed if you are using ebtables over nftables
@ -65,7 +65,7 @@ if BRIDGE_NF_EBTABLES
#
config BRIDGE_EBT_BROUTE
tristate "ebt: broute table support"
select BRIDGE_NF_EBTABLES_LEGACY
depends on BRIDGE_NF_EBTABLES_LEGACY
help
The ebtables broute table is used to define rules that decide between
bridging and routing frames, giving Linux the functionality of a
@ -76,7 +76,7 @@ config BRIDGE_EBT_BROUTE
config BRIDGE_EBT_T_FILTER
tristate "ebt: filter table support"
select BRIDGE_NF_EBTABLES_LEGACY
depends on BRIDGE_NF_EBTABLES_LEGACY
help
The ebtables filter table is used to define frame filtering rules at
local input, forwarding and local output. See the man page for
@ -86,7 +86,7 @@ config BRIDGE_EBT_T_FILTER
config BRIDGE_EBT_T_NAT
tristate "ebt: nat table support"
select BRIDGE_NF_EBTABLES_LEGACY
depends on BRIDGE_NF_EBTABLES_LEGACY
help
The ebtables nat table is used to define rules that alter the MAC
source address (MAC SNAT) or the MAC destination address (MAC DNAT).

24
net/ipv4/netfilter/Kconfig
View file

@ -13,8 +13,8 @@ config NF_DEFRAG_IPV4
# old sockopt interface and eval loop
config IP_NF_IPTABLES_LEGACY
tristate "Legacy IP tables support"
default n
select NETFILTER_XTABLES
depends on NETFILTER_XTABLES_LEGACY
default m if NETFILTER_XTABLES_LEGACY
help
iptables is a legacy packet classifier.
This is not needed if you are using iptables over nftables
@ -182,8 +182,8 @@ config IP_NF_MATCH_TTL
# `filter', generic and specific targets
config IP_NF_FILTER
tristate "Packet filtering"
default m if NETFILTER_ADVANCED=n
select IP_NF_IPTABLES_LEGACY
default m if NETFILTER_ADVANCED=n || IP_NF_IPTABLES_LEGACY
depends on IP_NF_IPTABLES_LEGACY
help
Packet filtering defines a table `filter', which has a series of
rules for simple packet filtering at local input, forwarding and
@ -220,10 +220,10 @@ config IP_NF_TARGET_SYNPROXY
config IP_NF_NAT
tristate "iptables NAT support"
depends on NF_CONNTRACK
depends on IP_NF_IPTABLES_LEGACY
default m if NETFILTER_ADVANCED=n
select NF_NAT
select NETFILTER_XT_NAT
select IP_NF_IPTABLES_LEGACY
help
This enables the `nat' table in iptables. This allows masquerading,
port forwarding and other forms of full Network Address Port
@ -263,8 +263,8 @@ endif # IP_NF_NAT
# mangle + specific targets
config IP_NF_MANGLE
tristate "Packet mangling"
default m if NETFILTER_ADVANCED=n
select IP_NF_IPTABLES_LEGACY
default m if NETFILTER_ADVANCED=n || IP_NF_IPTABLES_LEGACY
depends on IP_NF_IPTABLES_LEGACY
help
This option adds a `mangle' table to iptables: see the man page for
iptables(8). This table is used for various packet alterations
@ -299,7 +299,7 @@ config IP_NF_TARGET_TTL
# raw + specific targets
config IP_NF_RAW
tristate 'raw table support (required for NOTRACK/TRACE)'
select IP_NF_IPTABLES_LEGACY
depends on IP_NF_IPTABLES_LEGACY
help
This option adds a `raw' table to iptables. This table is the very
first in the netfilter framework and hooks in at the PREROUTING
@ -313,7 +313,7 @@ config IP_NF_SECURITY
tristate "Security table"
depends on SECURITY
depends on NETFILTER_ADVANCED
select IP_NF_IPTABLES_LEGACY
depends on IP_NF_IPTABLES_LEGACY
help
This option adds a `security' table to iptables, for use
with Mandatory Access Control (MAC) policy.
@ -325,8 +325,8 @@ endif # IP_NF_IPTABLES
# ARP tables
config IP_NF_ARPTABLES
tristate "Legacy ARPTABLES support"
depends on NETFILTER_XTABLES
default n
depends on NETFILTER_XTABLES_LEGACY
default n
help
arptables is a legacy packet classifier.
This is not needed if you are using arptables over nftables
@ -342,7 +342,7 @@ config IP_NF_ARPFILTER
tristate "arptables-legacy packet filtering support"
select IP_NF_ARPTABLES
select NETFILTER_FAMILY_ARP
depends on NETFILTER_XTABLES
depends on NETFILTER_XTABLES_LEGACY
help
ARP packet filtering defines a table `filter', which has a series of
rules for simple ARP packet filtering at local input and

19
net/ipv6/netfilter/Kconfig
View file

@ -9,9 +9,8 @@ menu "IPv6: Netfilter Configuration"
# old sockopt interface and eval loop
config IP6_NF_IPTABLES_LEGACY
tristate "Legacy IP6 tables support"
depends on INET && IPV6
select NETFILTER_XTABLES
default n
depends on INET && IPV6 && NETFILTER_XTABLES_LEGACY
default m if NETFILTER_XTABLES_LEGACY
help
ip6tables is a legacy packet classifier.
This is not needed if you are using iptables over nftables
@ -196,8 +195,8 @@ config IP6_NF_TARGET_HL
config IP6_NF_FILTER
tristate "Packet filtering"
default m if NETFILTER_ADVANCED=n
select IP6_NF_IPTABLES_LEGACY
default m if NETFILTER_ADVANCED=n || IP6_NF_IPTABLES_LEGACY
depends on IP6_NF_IPTABLES_LEGACY
tristate
help
Packet filtering defines a table `filter', which has a series of
@ -233,8 +232,8 @@ config IP6_NF_TARGET_SYNPROXY
config IP6_NF_MANGLE
tristate "Packet mangling"
default m if NETFILTER_ADVANCED=n
select IP6_NF_IPTABLES_LEGACY
default m if NETFILTER_ADVANCED=n || IP6_NF_IPTABLES_LEGACY
depends on IP6_NF_IPTABLES_LEGACY
help
This option adds a `mangle' table to iptables: see the man page for
iptables(8). This table is used for various packet alterations
@ -244,7 +243,7 @@ config IP6_NF_MANGLE
config IP6_NF_RAW
tristate 'raw table support (required for TRACE)'
select IP6_NF_IPTABLES_LEGACY
depends on IP6_NF_IPTABLES_LEGACY
help
This option adds a `raw' table to ip6tables. This table is the very
first in the netfilter framework and hooks in at the PREROUTING
@ -258,7 +257,7 @@ config IP6_NF_SECURITY
tristate "Security table"
depends on SECURITY
depends on NETFILTER_ADVANCED
select IP6_NF_IPTABLES_LEGACY
depends on IP6_NF_IPTABLES_LEGACY
help
This option adds a `security' table to iptables, for use
with Mandatory Access Control (MAC) policy.
@ -269,8 +268,8 @@ config IP6_NF_NAT
tristate "ip6tables NAT support"
depends on NF_CONNTRACK
depends on NETFILTER_ADVANCED
depends on IP6_NF_IPTABLES_LEGACY
select NF_NAT
select IP6_NF_IPTABLES_LEGACY
select NETFILTER_XT_NAT
help
This enables the `nat' table in ip6tables. This allows masquerading,

10
net/netfilter/Kconfig
View file

@ -758,6 +758,16 @@ config NETFILTER_XTABLES_COMPAT
If unsure, say N.
config NETFILTER_XTABLES_LEGACY
bool "Netfilter legacy tables support"
depends on !PREEMPT_RT
help
Say Y here if you still require support for legacy tables. This is
required by the legacy tools (iptables-legacy) and is not needed if
you use iptables over nftables (iptables-nft).
Legacy support is not limited to IP, it also includes EBTABLES and
ARPTABLES.
comment "Xtables combined modules"
config NETFILTER_XT_MARK

16
net/netfilter/x_tables.c
View file

@ -1317,12 +1317,13 @@ void xt_compat_unlock(u_int8_t af)
EXPORT_SYMBOL_GPL(xt_compat_unlock);
#endif
DEFINE_PER_CPU(seqcount_t, xt_recseq);
EXPORT_PER_CPU_SYMBOL_GPL(xt_recseq);
struct static_key xt_tee_enabled __read_mostly;
EXPORT_SYMBOL_GPL(xt_tee_enabled);
#ifdef CONFIG_NETFILTER_XTABLES_LEGACY
DEFINE_PER_CPU(seqcount_t, xt_recseq);
EXPORT_PER_CPU_SYMBOL_GPL(xt_recseq);
static int xt_jumpstack_alloc(struct xt_table_info *i)
{
unsigned int size;
@ -1514,6 +1515,7 @@ void *xt_unregister_table(struct xt_table *table)
return private;
}
EXPORT_SYMBOL_GPL(xt_unregister_table);
#endif
#ifdef CONFIG_PROC_FS
static void *xt_table_seq_start(struct seq_file *seq, loff_t *pos)
@ -1897,6 +1899,7 @@ void xt_proto_fini(struct net *net, u_int8_t af)
}
EXPORT_SYMBOL_GPL(xt_proto_fini);
#ifdef CONFIG_NETFILTER_XTABLES_LEGACY
/**
* xt_percpu_counter_alloc - allocate x_tables rule counter
*
@ -1951,6 +1954,7 @@ void xt_percpu_counter_free(struct xt_counters *counters)
free_percpu((void __percpu *)pcnt);
}
EXPORT_SYMBOL_GPL(xt_percpu_counter_free);
#endif
static int __net_init xt_net_init(struct net *net)
{
@ -1983,8 +1987,10 @@ static int __init xt_init(void)
unsigned int i;
int rv;
for_each_possible_cpu(i) {
seqcount_init(&per_cpu(xt_recseq, i));
if (IS_ENABLED(CONFIG_NETFILTER_XTABLES_LEGACY)) {
for_each_possible_cpu(i) {
seqcount_init(&per_cpu(xt_recseq, i));
}
}
xt = kcalloc(NFPROTO_NUMPROTO, sizeof(struct xt_af), GFP_KERNEL);