CVE-2021-38209: Observable Discrepancy (CWE-203)
value: net/netfilter/nf_conntrack_standalone.c in the Linux kernel before 5.12.2 allows observation of changes in any net namespace because these changes are leaked into all other net namespaces. This is related to the NF_SYSCTL_CT_MAX, NF_SYSCTL_CT_EXPECT_MAX, and NF_SYSCTL_CT_BUCKETS sysctls.
CWE description: The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.
CVSS v3 base score: 3.3
Published: 2021-08-08T20:15Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/netfilter/nf_conntrack_standalone.c
Commit date: 2021-04-12 13:27:11 -0700
Commit message: netfilter: conntrack: Make global sysctls readonly in non-init netns

These sysctls point to global variables:
- NF_SYSCTL_CT_MAX (&nf_conntrack_max)
- NF_SYSCTL_CT_EXPECT_MAX (&nf_ct_expect_max)
- NF_SYSCTL_CT_BUCKETS (&nf_conntrack_htable_size_user)

Because their data pointers are not updated to point to per-netns
structures, they must be marked read-only in a non-init_net ns.
Otherwise, changes in any net namespace are reflected in (leaked into)
all other net namespaces. This problem has existed since the
introduction of net namespaces.

The current logic marks them read-only only if the net namespace is
owned by an unprivileged user (other than init_user_ns).

Commit d0febd81ae77 ("netfilter: conntrack: re-visit sysctls in
unprivileged namespaces") "exposes all sysctls even if the namespace is
unpriviliged." Since we need to mark them readonly in any case, we can
forego the unprivileged user check altogether.

Fixes: d0febd81ae77 ("netfilter: conntrack: re-visit sysctls in unprivileged namespaces")
Signed-off-by: Jonathon Reinhart <Jonathon.Reinhart@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/netfilter/nf_conntrack_standalone.c b/net/netfilter/nf_conntrack_standalone.c
index 3f2cc7b04b20..54d36d3eb905 100644
--- a/net/netfilter/nf_conntrack_standalone.c
+++ b/net/netfilter/nf_conntrack_standalone.c
@@ -1058,20 +1058,14 @@ static int nf_conntrack_standalone_init_sysctl(struct net *net)
 	nf_conntrack_standalone_init_tcp_sysctl(net, table);
 	nf_conntrack_standalone_init_sctp_sysctl(net, table);
 	nf_conntrack_standalone_init_dccp_sysctl(net, table);
 	nf_conntrack_standalone_init_gre_sysctl(net, table);
 
-	/* Don't allow unprivileged users to alter certain sysctls */
-	if (net->user_ns != &init_user_ns) {
+	/* Don't allow non-init_net ns to alter global sysctls */
+	if (!net_eq(&init_net, net)) {
 		table[NF_SYSCTL_CT_MAX].mode = 0444;
 		table[NF_SYSCTL_CT_EXPECT_MAX].mode = 0444;
-		table[NF_SYSCTL_CT_HELPER].mode = 0444;
-#ifdef CONFIG_NF_CONNTRACK_EVENTS
-		table[NF_SYSCTL_CT_EVENTS].mode = 0444;
-#endif
-		table[NF_SYSCTL_CT_BUCKETS].mode = 0444;
-	} else if (!net_eq(&init_net, net)) {
 		table[NF_SYSCTL_CT_BUCKETS].mode = 0444;
 	}
 
 	cnet->sysctl_header = register_net_sysctl(net, "net/netfilter", table);
 	if (!cnet->sysctl_header)

Vulnerable code: 	/* Don't allow unprivileged users to alter certain sysctls */
	if (net->user_ns != &init_user_ns) {
		table[NF_SYSCTL_CT_HELPER].mode = 0444;
#ifdef CONFIG_NF_CONNTRACK_EVENTS
		table[NF_SYSCTL_CT_EVENTS].mode = 0444;
#endif
		table[NF_SYSCTL_CT_BUCKETS].mode = 0444;
	} else if (!net_eq(&init_net, net)) {
Fixed code: 	/* Don't allow non-init_net ns to alter global sysctls */
	if (!net_eq(&init_net, net)) {