[PATCH net] ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

[Norbert Szetei]

pppol2tp_recv() runs in the L2TP UDP-encap softirq RX path:

 l2tp_udp_encap_recv() -> l2tp_recv_common() -> pppol2tp_recv()
   -> ppp_input(&po->chan)

It runs under rcu_read_lock() holding only an l2tp_session reference and
takes NO reference on the internal PPP channel (struct channel,
chan->ppp) that ppp_input() dereferences.

The pppox socket is SOCK_RCU_FREE, so 'po' and the embedded ppp_channel
are RCU-safe.  But the internal struct channel is a separate allocation
that ppp_release_channel() frees with a plain kfree():

 close(data socket) -> pppol2tp_release() -> pppox_unbind_sock()
   -> ppp_unregister_channel() -> ppp_release_channel() -> kfree(pch)

For a channel that is bound (PPPIOCGCHAN) but not attached to a ppp unit
(no PPPIOCCONNECT, pch->ppp == NULL) and not bridged, teardown skips
both ppp_disconnect_channel()'s synchronize_net() and
ppp_unbridge_channels()'s synchronize_rcu(), so the kfree() has no grace
period.  rcu_read_lock() in pppol2tp_recv() does not protect against a
plain kfree(), so an in-flight ppp_input() on one CPU can dereference
the channel just freed by close() on another CPU.

The bug is reachable by an unprivileged user.

Free the channel with kfree_rcu() instead of kfree() so the grace period
fences any in-flight ppp_input(). Done in ppp_release_channel(), this
covers all callers in one place.

Fixes: ee40fb2e1eb5 ("l2tp: protect sock pointer of struct pppol2tp_session with RCU")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Norbert Szetei <norbert@xxxxxxxxxxxx>
---
 drivers/net/ppp/ppp_generic.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 57c68efa5ff8..cb8fe37170d3 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -184,6 +184,7 @@ struct channel {
 	struct list_head clist;		/* link in list of channels per unit */
 	spinlock_t	upl;		/* protects `ppp' and 'bridge' */
 	struct channel __rcu *bridge;	/* "bridged" ppp channel */
+	struct rcu_head	rcu;		/* for RCU-deferred free of the channel */
 #ifdef CONFIG_PPP_MULTILINK
 	u8		avail;		/* flag used in multilink stuff */
 	u8		had_frag;	/* >= 1 fragments have been sent */
@@ -3583,7 +3584,7 @@ static void ppp_release_channel(struct channel *pch)
 	}
 	skb_queue_purge(&pch->file.xq);
 	skb_queue_purge(&pch->file.rq);
-	kfree(pch);
+	kfree_rcu(pch, rcu);
 }
 
 static void __exit ppp_cleanup(void)
-- 
2.54.0