[PATCH net] seg6: fix tun_src RCU lifetime on namespace teardown

[Tristan Madani <tristmd@gmail.com>]

From: Tristan Madani <tristan@talencesecurity.com>

seg6_net_exit() frees tun_src directly, but set_tun_src() in
seg6_iptunnel.c reads it under rcu_read_lock(). The pointer may
be freed while a concurrent reader is still accessing it.

Add a pre_exit callback to clear the tun_src pointer before the
implied synchronize_rcu_expedited(), so all RCU readers complete
before the actual kfree in the exit callback. Save the original
pointer in tun_src_old so exit can free it after the grace period,
since the RCU pointer is already NULL at that point. The init
error path frees tun_src directly since pre_exit has not run.

Fixes: 915d7e5e5930 ("ipv6: sr: add code base for control plane support of SR-IPv6")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 include/net/seg6.h |  1 +
 net/ipv6/seg6.c    | 12 +++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/net/seg6.h b/include/net/seg6.h
index 82b3fbbcbb935..210f4fa0d4dd7 100644
--- a/include/net/seg6.h
+++ b/include/net/seg6.h
@@ -36,6 +36,7 @@ static inline void update_csum_diff16(struct sk_buff *skb, __be32 *from,
 struct seg6_pernet_data {
 	struct mutex lock;
 	struct in6_addr __rcu *tun_src;
+	struct in6_addr *tun_src_old;
 #ifdef CONFIG_IPV6_SEG6_HMAC
 	struct rhashtable hmac_infos;
 #endif
diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
index 8c2b156c227a7..670052290e43c 100644
--- a/net/ipv6/seg6.c
+++ b/net/ipv6/seg6.c
@@ -447,18 +447,28 @@ static int __net_init seg6_net_init(struct net *net)
 	return 0;
 }
 
+static void __net_exit seg6_net_pre_exit(struct net *net)
+{
+	struct seg6_pernet_data *sdata = seg6_pernet(net);
+
+	/* Save tun_src for exit to free after the RCU grace period. */
+	sdata->tun_src_old = rcu_dereference_raw(sdata->tun_src);
+	RCU_INIT_POINTER(sdata->tun_src, NULL);
+}
+
 static void __net_exit seg6_net_exit(struct net *net)
 {
 	struct seg6_pernet_data *sdata = seg6_pernet(net);
 
 	seg6_hmac_net_exit(net);
 
-	kfree(rcu_dereference_raw(sdata->tun_src));
+	kfree(sdata->tun_src_old);
 	kfree(sdata);
 }
 
 static struct pernet_operations ip6_segments_ops = {
 	.init = seg6_net_init,
+	.pre_exit = seg6_net_pre_exit,
 	.exit = seg6_net_exit,
 };
 
-- 
2.47.3