[PATCH net 1/6] net: openvswitch: conntrack: avoid modifying shared unconfirmed ct entry

[Ilya Maximets]

In a case where skb with an unconfirmed ct entry gets cloned, we may
end up committing both but with different sets of extensions.

The series of events:

 1. The first clone wants to commit and runs the helpers wiring up
    the extension pointer into the expectation list.
 2. Then it looses the confirmation keeping the entry unconfirmed.
 3. Second clone now wants to commit labels and adds the new extension
    for that breaking the pointer in the expectation list causing
    UAF on the destruction path later.

While this is possible to trigger, there should be no practical
network pipeline where committing both clones without modifications
into the same zone is needed.  So, let's just reset the entry in case
for some reason we got an skb with a shared one during commit.  This
doesn't affect any known use cases, but avoids any potential problems
with sharing and modification of the unconfirmed ct entry.

The fixes tag points to the introduction of helpers, since that's the
main UAF trigger for the sharing.

Fixes: cae3a2627520 ("openvswitch: Allow attaching helpers to ct action")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Axel Mierczuk <axel.mierczuk@xxxxxxxxxxxxx>
Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
---
 include/net/netfilter/nf_conntrack.h |  5 +++++
 net/openvswitch/conntrack.c          | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index bc42dd0e10e65..c39425e54d87d 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -185,6 +185,11 @@ static inline void nf_ct_put(struct nf_conn *ct)
 		nf_ct_destroy(&ct->ct_general);
 }
 
+static inline bool nf_ct_shared(const struct nf_conn *ct)
+{
+	return refcount_read(&ct->ct_general.use) > 1;
+}
+
 /* load module; enable/disable conntrack in this namespace */
 int nf_ct_netns_get(struct net *net, u8 nfproto);
 void nf_ct_netns_put(struct net *net, u8 nfproto);
diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index 0f433688e17b9..a733029c28dd0 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -734,6 +734,18 @@ static int __ovs_ct_lookup(struct net *net, struct sw_flow_key *key,
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct;
 
+	/* If the ct entry is not confirmed and shared with some other skb,
+	 * e.g., a cloned one, we can't just modify it with the commit as we
+	 * must not modify the extension set.  Reset.
+	 */
+	if (cached && info->commit) {
+		ct = nf_ct_get(skb, &ctinfo);
+		if (ct && !nf_ct_is_confirmed(ct) && nf_ct_shared(ct)) {
+			nf_reset_ct(skb);
+			cached = false;
+		}
+	}
+
 	if (!cached) {
 		struct nf_hook_state state = {
 			.hook = NF_INET_PRE_ROUTING,
-- 
2.55.0