Revert "net: Remove low_thresh in ip defrag"

This reverts commit b2cbac9.

We have multiple reports of obvious breakage from this patch.

Reported-by: Ido Schimmel <idosch@idosch.org>
Link: https://lore.kernel.org/all/ZGIRWjNcfqI8yY8W@shredder/
Link: https://lore.kernel.org/all/CADJHv_sDK=0RrMA2FTZQV5fw7UQ+qY=HG21Wu5qb0V9vvx5w6A@mail.gmail.com/
Reported-by: syzbot+a5e719ac7c268e414c95@syzkaller.appspotmail.com
Reported-by: syzbot+a03fd670838d927d9cd8@syzkaller.appspotmail.com
Fixes: b2cbac9 ("net: Remove low_thresh in ip defrag")
Link: https://lore.kernel.org/r/20230517034112.1261835-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/nf_conntrack-sysctl.rst

@@ -55,7 +55,6 @@ nf_conntrack_frag6_high_thresh - INTEGER
 	nf_conntrack_frag6_low_thresh is reached.
 
 nf_conntrack_frag6_low_thresh - INTEGER
-	(Obsolete since linux-4.17)
 	default 196608
 
 	See nf_conntrack_frag6_low_thresh

include/net/inet_frag.h

@@ -13,6 +13,7 @@
 struct fqdir {
 	/* sysctls */
 	long			high_thresh;
+	long			low_thresh;
 	int			timeout;
 	int			max_dist;
 	struct inet_frags	*f;

net/ieee802154/6lowpan/reassembly.c

@@ -318,7 +318,7 @@ int lowpan_frag_rcv(struct sk_buff *skb, u8 frag_type)
 }
 
 #ifdef CONFIG_SYSCTL
-static unsigned long lowpanfrag_low_thresh_unuesd = IPV6_FRAG_LOW_THRESH;
+
 static struct ctl_table lowpan_frags_ns_ctl_table[] = {
 	{
 		.procname	= "6lowpanfrag_high_thresh",
@@ -374,9 +374,9 @@ static int __net_init lowpan_frags_ns_sysctl_register(struct net *net)
 	}
 
 	table[0].data	= &ieee802154_lowpan->fqdir->high_thresh;
-	table[0].extra1 = &lowpanfrag_low_thresh_unuesd;
-	table[1].data   = &lowpanfrag_low_thresh_unuesd;
-	table[1].extra2 = &ieee802154_lowpan->fqdir->high_thresh;
+	table[0].extra1	= &ieee802154_lowpan->fqdir->low_thresh;
+	table[1].data	= &ieee802154_lowpan->fqdir->low_thresh;
+	table[1].extra2	= &ieee802154_lowpan->fqdir->high_thresh;
 	table[2].data	= &ieee802154_lowpan->fqdir->timeout;
 
 	hdr = register_net_sysctl(net, "net/ieee802154/6lowpan", table);
@@ -451,6 +451,7 @@ static int __net_init lowpan_frags_init_net(struct net *net)
 		return res;
 
 	ieee802154_lowpan->fqdir->high_thresh = IPV6_FRAG_HIGH_THRESH;
+	ieee802154_lowpan->fqdir->low_thresh = IPV6_FRAG_LOW_THRESH;
 	ieee802154_lowpan->fqdir->timeout = IPV6_FRAG_TIMEOUT;
 
 	res = lowpan_frags_ns_sysctl_register(net);

net/ipv4/ip_fragment.c

@@ -553,7 +553,7 @@ EXPORT_SYMBOL(ip_check_defrag);
 
 #ifdef CONFIG_SYSCTL
 static int dist_min;
-static unsigned long ipfrag_low_thresh_unused;
+
 static struct ctl_table ip4_frags_ns_ctl_table[] = {
 	{
 		.procname	= "ipfrag_high_thresh",
@@ -609,9 +609,9 @@ static int __net_init ip4_frags_ns_ctl_register(struct net *net)
 
 	}
 	table[0].data	= &net->ipv4.fqdir->high_thresh;
-	table[0].extra1 = &ipfrag_low_thresh_unused;
-	table[1].data	= &ipfrag_low_thresh_unused;
-	table[1].extra2 = &net->ipv4.fqdir->high_thresh;
+	table[0].extra1	= &net->ipv4.fqdir->low_thresh;
+	table[1].data	= &net->ipv4.fqdir->low_thresh;
+	table[1].extra2	= &net->ipv4.fqdir->high_thresh;
 	table[2].data	= &net->ipv4.fqdir->timeout;
 	table[3].data	= &net->ipv4.fqdir->max_dist;
 
@@ -674,9 +674,12 @@ static int __net_init ipv4_frags_init_net(struct net *net)
 	 * A 64K fragment consumes 129736 bytes (44*2944)+200
 	 * (1500 truesize == 2944, sizeof(struct ipq) == 200)
 	 *
-	 * We will commit 4MB at one time. Should we cross that limit.
+	 * We will commit 4MB at one time. Should we cross that limit
+	 * we will prune down to 3MB, making room for approx 8 big 64K
+	 * fragments 8x128k.
 	 */
 	net->ipv4.fqdir->high_thresh = 4 * 1024 * 1024;
+	net->ipv4.fqdir->low_thresh  = 3 * 1024 * 1024;
 	/*
 	 * Important NOTE! Fragment queue must be destroyed before MSL expires.
 	 * RFC791 is wrong proposing to prolongate timer each fragment arrival

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -42,7 +42,7 @@ static struct nft_ct_frag6_pernet *nf_frag_pernet(struct net *net)
 }
 
 #ifdef CONFIG_SYSCTL
-static unsigned long nf_conntrack_frag6_low_thresh_unused = IPV6_FRAG_LOW_THRESH;
+
 static struct ctl_table nf_ct_frag6_sysctl_table[] = {
 	{
 		.procname	= "nf_conntrack_frag6_timeout",
@@ -82,10 +82,10 @@ static int nf_ct_frag6_sysctl_register(struct net *net)
 	nf_frag = nf_frag_pernet(net);
 
 	table[0].data	= &nf_frag->fqdir->timeout;
-	table[1].data	= &nf_conntrack_frag6_low_thresh_unused;
-	table[1].extra2 = &nf_frag->fqdir->high_thresh;
+	table[1].data	= &nf_frag->fqdir->low_thresh;
+	table[1].extra2	= &nf_frag->fqdir->high_thresh;
 	table[2].data	= &nf_frag->fqdir->high_thresh;
-	table[2].extra1 = &nf_conntrack_frag6_low_thresh_unused;
+	table[2].extra1	= &nf_frag->fqdir->low_thresh;
 
 	hdr = register_net_sysctl(net, "net/netfilter", table);
 	if (hdr == NULL)
@@ -500,6 +500,7 @@ static int nf_ct_net_init(struct net *net)
 		return res;
 
 	nf_frag->fqdir->high_thresh = IPV6_FRAG_HIGH_THRESH;
+	nf_frag->fqdir->low_thresh = IPV6_FRAG_LOW_THRESH;
 	nf_frag->fqdir->timeout = IPV6_FRAG_TIMEOUT;
 
 	res = nf_ct_frag6_sysctl_register(net);

net/ipv6/reassembly.c

@@ -416,7 +416,7 @@ static const struct inet6_protocol frag_protocol = {
 };
 
 #ifdef CONFIG_SYSCTL
-static unsigned long ip6_frags_low_thresh_unused = IPV6_FRAG_LOW_THRESH;
+
 static struct ctl_table ip6_frags_ns_ctl_table[] = {
 	{
 		.procname	= "ip6frag_high_thresh",
@@ -465,9 +465,9 @@ static int __net_init ip6_frags_ns_sysctl_register(struct net *net)
 
 	}
 	table[0].data	= &net->ipv6.fqdir->high_thresh;
-	table[0].extra1 = &ip6_frags_low_thresh_unused;
-	table[1].data   = &ip6_frags_low_thresh_unused;
-	table[1].extra2 = &net->ipv6.fqdir->high_thresh;
+	table[0].extra1	= &net->ipv6.fqdir->low_thresh;
+	table[1].data	= &net->ipv6.fqdir->low_thresh;
+	table[1].extra2	= &net->ipv6.fqdir->high_thresh;
 	table[2].data	= &net->ipv6.fqdir->timeout;
 
 	hdr = register_net_sysctl(net, "net/ipv6", table);
@@ -536,6 +536,7 @@ static int __net_init ipv6_frags_init_net(struct net *net)
 		return res;
 
 	net->ipv6.fqdir->high_thresh = IPV6_FRAG_HIGH_THRESH;
+	net->ipv6.fqdir->low_thresh = IPV6_FRAG_LOW_THRESH;
 	net->ipv6.fqdir->timeout = IPV6_FRAG_TIMEOUT;
 
 	res = ip6_frags_ns_sysctl_register(net);