RE: [PATCH net v2] tipc: fix use-after-free of the discoverer in tipc_disc_rcv()

[Tung Quang Nguyen]

>Subject: [PATCH net v2] tipc: fix use-after-free of the discoverer in
>tipc_disc_rcv()
>
>bearer_disable() frees b->disc with tipc_disc_delete()'s plain kfree(), but
>tipc_disc_rcv() still dereferences b->disc in RX softirq under
>rcu_read_lock() (tipc_udp_recv -> tipc_rcv -> tipc_disc_rcv).
>
>L2 bearers are safe thanks to the synchronize_net() in tipc_disable_l2_media(),
>but the UDP bearer defers that call to the
>cleanup_bearer() workqueue, so the discoverer is freed with no grace
>period:
>
> BUG: KASAN: slab-use-after-free in tipc_disc_rcv (net/tipc/discover.c:149)
>Read of size 8 at addr ffff88802348b728 by task poc_tipc/184  <IRQ>
>  tipc_disc_rcv (net/tipc/discover.c:149)
>  tipc_rcv (net/tipc/node.c:2126)
>  tipc_udp_recv (net/tipc/udp_media.c:391)
>  udp_rcv (net/ipv4/udp.c:2643)
>  ip_local_deliver_finish (net/ipv4/ip_input.c:241)  </IRQ>  Freed by task 181:
>  kfree (mm/slub.c:6565)
>  bearer_disable (net/tipc/bearer.c:418)
>  tipc_nl_bearer_disable (net/tipc/bearer.c:1001)
>
>The bearer is freed with kfree_rcu(); free the discoverer the same way.
>Add an rcu_head to struct tipc_discoverer and free it and its skb from an RCU
>callback.
>
>Because the RCU callback (tipc_disc_free_rcu) lives in module text, a
>call_rcu() that is still pending when the tipc module is unloaded would invoke a
>freed function. Add an rcu_barrier() to tipc_exit() after the bearer subsystem
>has been torn down, so all pending discoverer callbacks have run before the
>module text goes away.
>
>Reachable from an unprivileged user namespace: the TIPCv2 genl family is
>netnsok and its bearer commands have no GENL_ADMIN_PERM. Needs
>CONFIG_TIPC and CONFIG_TIPC_MEDIA_UDP.
>
>Fixes: 25b0b9c4e835 ("tipc: handle collisions of 32-bit node address hash
>values")
>Reported-by: Xiang Mei <xmei5@xxxxxxx>
>Assisted-by: Claude:claude-opus-4-8
>Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>
>---
>v2:
> - split the over-80-column container_of() line (Tung Quang Nguyen)
> - add rcu_barrier() to tipc_exit() so a pending call_rcu() cannot fire
>   into freed module text after rmmod (Eric Dumazet)
>
> net/tipc/core.c     |  3 +++
> net/tipc/discover.c | 14 ++++++++++++--
> 2 files changed, 15 insertions(+), 2 deletions(-)
>
>diff --git a/net/tipc/core.c b/net/tipc/core.c index
>434e70eabe08..747328e58d30 100644
>--- a/net/tipc/core.c
>+++ b/net/tipc/core.c
>@@ -218,6 +218,9 @@ static void __exit tipc_exit(void)
> 	unregister_pernet_device(&tipc_net_ops);
> 	tipc_unregister_sysctl();
>
>+	/* Wait for tipc_disc_free_rcu() callbacks queued from module text. */

Please change above comment to: /* TODO: Wait for all timers that  called call_rcu() to finish before calling rcu_barrier() */

Note that call_rcu() are used in discover.c and node.c. So, the TODO comment helps we add more checking code later in another patch.

>+	rcu_barrier();
>+
> 	pr_info("Deactivated\n");
> }
>
>diff --git a/net/tipc/discover.c b/net/tipc/discover.c index
>3e54d2df5683..696b7a8ed54d 100644
>--- a/net/tipc/discover.c
>+++ b/net/tipc/discover.c
>@@ -58,6 +58,7 @@
>  * @skb: request message to be (repeatedly) sent
>  * @timer: timer governing period between requests
>  * @timer_intv: current interval between requests (in ms)
>+ * @rcu: RCU head for deferred freeing
>  */
> struct tipc_discoverer {
> 	u32 bearer_id;
>@@ -69,6 +70,7 @@ struct tipc_discoverer {
> 	struct sk_buff *skb;
> 	struct timer_list timer;
> 	unsigned long timer_intv;
>+	struct rcu_head rcu;
> };
>
> /**
>@@ -382,6 +384,15 @@ int tipc_disc_create(struct net *net, struct tipc_bearer
>*b,
> 	return 0;
> }
>
>+static void tipc_disc_free_rcu(struct rcu_head *rp) {
>+	struct tipc_discoverer *d =
>+		container_of(rp, struct tipc_discoverer, rcu);
>+
>+	kfree_skb(d->skb);
>+	kfree(d);
>+}
>+
> /**
>  * tipc_disc_delete - destroy object sending periodic link setup requests
>  * @d: ptr to link dest structure
>@@ -389,8 +400,7 @@ int tipc_disc_create(struct net *net, struct tipc_bearer
>*b,  void tipc_disc_delete(struct tipc_discoverer *d)  {
> 	timer_shutdown_sync(&d->timer);
>-	kfree_skb(d->skb);
>-	kfree(d);
>+	call_rcu(&d->rcu, tipc_disc_free_rcu);
> }
>
> /**
>--
>2.43.0