[PATCH net] sctp: re-point the GSO head_skb socket on association migration

[Jun Yang <juny24602@gmail.com>]

From: Jun Yang <junvyyang@tencent.com>

sctp_ulpevent_set_owner() stashes the receiving socket on the GSO "cover
letter" skb as a bare pointer (net/sctp/ulpevent.c:90):

	if (chunk && chunk->head_skb && !chunk->head_skb->sk)
		chunk->head_skb->sk = asoc->base.sk;

That store takes no reference and installs no destructor, unlike the
sctp_skb_set_owner_r() applied to the event skb just above it.

When an association is moved to another socket -- SCTP_SOCKOPT_PEELOFF, or
accept() on a TCP-style socket -- sctp_sock_migrate() re-owns the queued
skbs with sctp_skb_set_owner_r_frag().  That walks the receive, lobby and
reassembly queues, but chunk->head_skb is on none of them: it is reachable
only through event->chunk->head_skb.  Its ->sk therefore keeps pointing at
the original socket.  Once that socket is closed and freed, the dangling
pointer is dereferenced on the very next receive, in sctp_recvmsg()
(net/sctp/socket.c:2155):

	sp->pf->skb_msgname(head_skb, msg->msg_name, &msg->msg_namelen);

which for IPv6 reads sctp_sk(skb->sk)->v4mapped (net/sctp/ipv6.c:894).

Re-point the cover-letter skb during migration as well.  The event lives in
skb->cb of the event skb only, so this cannot be folded into
sctp_skb_set_owner_r_frag(), which also recurses over fragment skbs; add a
small wrapper and use it at the three migration call sites.

  BUG: KASAN: slab-use-after-free in sctp_inet6_skb_msgname+0x633/0xb30
  Read of size 2 at addr ff1100010aafc8e0 by task gso_uaf/6387
  CPU: 1 UID: 1000 PID: 6387 Comm: gso_uaf
   sctp_inet6_skb_msgname+0x633/0xb30
   sctp_recvmsg+0x481/0xa00
   sock_recvmsg+0x121/0x170
  Freed by task 0:
   __sk_destruct+0x39d/0x4c0
   sctp_endpoint_destroy_rcu+0x8b/0xc0
  The buggy address belongs to the object at ff1100010aafc380
   which belongs to the cache SCTPv6 of size 1560

Fixes: 90017accff61 ("sctp: Add GSO support")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Signed-off-by: Jun Yang <junvyyang@tencent.com>
---
 net/sctp/socket.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index c7b9e325ec1c..ade413bbec56 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -9522,6 +9522,27 @@ static void sctp_skb_set_owner_r_frag(struct sk_buff *skb, struct sock *sk)
 	sctp_skb_set_owner_r(skb, sk);
 }
 
+/* Re-own a queued event's skbs, including the GSO "cover letter" skb.
+ *
+ * sctp_ulpevent_set_owner() stashes the socket on chunk->head_skb as a bare
+ * pointer, with no reference and no destructor.  That skb is not on any of the
+ * queues walked during a migration, so it has to be re-pointed explicitly.
+ *
+ * Only the event skb itself carries a struct sctp_ulpevent in ->cb, so this
+ * must not be folded into the recursive helper above, which also visits
+ * fragment skbs.
+ */
+static void sctp_skb_set_owner_r_event(struct sk_buff *skb, struct sock *sk)
+{
+	struct sctp_ulpevent *event = sctp_skb2event(skb);
+
+	sctp_skb_set_owner_r_frag(skb, sk);
+
+	if (event->chunk && event->chunk->head_skb &&
+	    event->chunk->head_skb != skb)
+		event->chunk->head_skb->sk = sk;
+}
+
 /* Populate the fields of the newsk from the oldsk and migrate the assoc
  * and its messages to the newsk.
  */
@@ -9571,7 +9592,7 @@ static int sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 		if (event->asoc == assoc) {
 			__skb_unlink(skb, &oldsk->sk_receive_queue);
 			__skb_queue_tail(&newsk->sk_receive_queue, skb);
-			sctp_skb_set_owner_r_frag(skb, newsk);
+			sctp_skb_set_owner_r_event(skb, newsk);
 		}
 	}
 
@@ -9600,7 +9621,7 @@ static int sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 			if (event->asoc == assoc) {
 				__skb_unlink(skb, &oldsp->pd_lobby);
 				__skb_queue_tail(queue, skb);
-				sctp_skb_set_owner_r_frag(skb, newsk);
+				sctp_skb_set_owner_r_event(skb, newsk);
 			}
 		}
 
@@ -9612,7 +9633,7 @@ static int sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 
 	}
 
-	sctp_for_each_rx_skb(assoc, newsk, sctp_skb_set_owner_r_frag);
+	sctp_for_each_rx_skb(assoc, newsk, sctp_skb_set_owner_r_event);
 
 	/* Set the type of socket to indicate that it is peeled off from the
 	 * original UDP-style socket or created with the accept() call on a
-- 
2.43.7