Kernel patch due to loss of helper

Posted by: Tuyen Pham Thanh 1 year, 3 months ago

(0 comments)

First of all, on the occasion of the lunar new year, I wish everyone good health.

The issue of a conntrack injection from user space lost helper that was mentioned in the expectation post. That's one way to fix it.

After discussion with security@kernel.org and concern parties, we agreed on the following patch:

diff --git a/include/uapi/linux/netfilter/nf_conntrack_common.h b/include/uapi/linux/netfilter/nf_conntrack_common.h
index 4b3395082d15..26071021e986 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_common.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_common.h
@@ -106,7 +106,7 @@ enum ip_conntrack_status {
 	IPS_NAT_CLASH = IPS_UNTRACKED,
 #endif
 
-	/* Conntrack got a helper explicitly attached via CT target. */
+	/* Conntrack got a helper explicitly attached (ruleset, ctnetlink). */
 	IPS_HELPER_BIT = 13,
 	IPS_HELPER = (1 << IPS_HELPER_BIT),
 
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index ac438370f94a..7032402ffd33 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -2311,7 +2311,8 @@ ctnetlink_create_conntrack(struct net *net,
 			if (helper->from_nlattr)
 				helper->from_nlattr(helpinfo, ct);
 
-			/* not in hash table yet so not strictly necessary */
+			/* disable helper auto-assignment for this entry */
+			ct->status |= IPS_HELPER;
 			RCU_INIT_POINTER(help->helper, helper);
 		}
 	} else {

*Update: Pablo Neira Ayuso, Netfilter project head, Linux kernel maintainer of the Netfilter subsystem, informed that the above patch has been applied.

[ Upstream commit d1ca60efc53d665cf89ed847a14a510a81770b81 ]

When userspace, e.g. conntrackd, inserts an entry with a specified helper,
its possible that the helper is lost immediately after its added:

ctnetlink_create_conntrack
  -> nf_ct_helper_ext_add + assign helper
    -> ctnetlink_setup_nat
      -> ctnetlink_parse_nat_setup
         -> parse_nat_setup -> nfnetlink_parse_nat_setup
	                       -> nf_nat_setup_info
                                 -> nf_conntrack_alter_reply
                                   -> __nf_ct_try_assign_helper

... and __nf_ct_try_assign_helper will zero the helper again.

Set IPS_HELPER bit to bypass auto-assign logic, its unwanted, just like
when helper is assigned via ruleset.

Dropped old 'not strictly necessary' comment, it referred to use of
rcu_assign_pointer() before it got replaced by RCU_INIT_POINTER().

NB: Fixes tag intentionally incorrect, this extends the referenced commit,
but this change won't build without IPS_HELPER introduced there.

Fixes: 6714cf5465d280 ("netfilter: nf_conntrack: fix explicit helper attachment and NAT")
Reported-by: Pham Thanh Tuyen <phamtyn@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/uapi/linux/netfilter/nf_conntrack_common.h | 2 +-
 net/netfilter/nf_conntrack_netlink.c               | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/netfilter/nf_conntrack_common.h b/include/uapi/linux/netfilter/nf_conntrack_common.h
index 4b3395082d15c..26071021e986f 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_common.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_common.h
@@ -106,7 +106,7 @@ enum ip_conntrack_status {
 	IPS_NAT_CLASH = IPS_UNTRACKED,
 #endif
 
-	/* Conntrack got a helper explicitly attached via CT target. */
+	/* Conntrack got a helper explicitly attached (ruleset, ctnetlink). */
 	IPS_HELPER_BIT = 13,
 	IPS_HELPER = (1 << IPS_HELPER_BIT),
 
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 81d03acf68d4d..1c02be04aaf5c 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -2310,7 +2310,8 @@ ctnetlink_create_conntrack(struct net *net,
 			if (helper->from_nlattr)
 				helper->from_nlattr(helpinfo, ct);
 
-			/* not in hash table yet so not strictly necessary */
+			/* disable helper auto-assignment for this entry */
+			ct->status |= IPS_HELPER;
 			RCU_INIT_POINTER(help->helper, helper);
 		}
 	} else {

• ← Building a fault-tolerant firewall system with virtual machines: Writing a firewall ruleset
• HVAC Security →