[PATCH net v2] net/smc: fix UAF in smc_cdc_rx_handler() by pinning the socket

[Xiang Mei <xmei5@asu.edu>]

smc_cdc_rx_handler() looks up the connection by token under the link
group's conns_lock, drops the lock, and then dereferences conn and the
smc_sock derived from it, ending in sock_hold(&smc->sk) inside
smc_cdc_msg_recv(). No reference is held across the lock release.

The only reference pinning the socket while the connection is
discoverable in the link group is taken in smc_lgr_register_conn()
(sock_hold) and dropped in __smc_lgr_unregister_conn() (sock_put), both
under conns_lock. Once the handler drops conns_lock, a concurrent
close() -> smc_release() -> smc_conn_free() -> smc_lgr_unregister_conn()
can drop that reference and free the smc_sock, so the handler's later
sock_hold() runs on freed memory:

  WARNING: lib/refcount.c:25 at refcount_warn_saturate
  Workqueue: rxe_wq do_work
   refcount_warn_saturate (lib/refcount.c:25)
   smc_cdc_msg_recv (net/smc/smc_cdc.c:430)
   smc_cdc_rx_handler (net/smc/smc_cdc.c:502)
   smc_wr_rx_tasklet_fn (net/smc/smc_wr.c:445)
   tasklet_action_common (kernel/softirq.c:938)
   handle_softirqs (kernel/softirq.c:622)
  Kernel panic - not syncing: panic_on_warn set

Only SMC-R is affected. The SMC-D receive tasklet is stopped by
tasklet_kill(&conn->rx_tsklet) in smc_conn_free() before the connection
is unregistered, so it cannot run concurrently with the free.

Take the socket reference while still holding conns_lock, so the
registration reference can no longer be the last one, and drop it once
the handler is done.

Fixes: d7b0e37c1ac1 ("net/smc: restructure CDC message reception")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
v2:
- Take the reference under conns_lock, and compute smc once
- Initialize smc = NULL at declaration

 net/smc/smc_cdc.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/net/smc/smc_cdc.c b/net/smc/smc_cdc.c
index 619b3bab3824..32d6d03df321 100644
--- a/net/smc/smc_cdc.c
+++ b/net/smc/smc_cdc.c
@@ -470,9 +470,9 @@ static void smc_cdc_rx_handler(struct ib_wc *wc, void *buf)
 {
 	struct smc_link *link = (struct smc_link *)wc->qp->qp_context;
 	struct smc_cdc_msg *cdc = buf;
+	struct smc_sock *smc = NULL;
 	struct smc_connection *conn;
 	struct smc_link_group *lgr;
-	struct smc_sock *smc;
 
 	if (wc->byte_len < offsetof(struct smc_cdc_msg, reserved))
 		return; /* short message */
@@ -483,21 +483,26 @@ static void smc_cdc_rx_handler(struct ib_wc *wc, void *buf)
 	lgr = smc_get_lgr(link);
 	read_lock_bh(&lgr->conns_lock);
 	conn = smc_lgr_find_conn(ntohl(cdc->token), lgr);
-	read_unlock_bh(&lgr->conns_lock);
-	if (!conn || conn->out_of_sync)
+	if (!conn || conn->out_of_sync) {
+		read_unlock_bh(&lgr->conns_lock);
 		return;
+	}
 	smc = container_of(conn, struct smc_sock, conn);
+	sock_hold(&smc->sk);
+	read_unlock_bh(&lgr->conns_lock);
 
 	if (cdc->prod_flags.failover_validation) {
 		smc_cdc_msg_validate(smc, cdc, link);
-		return;
+		goto out;
 	}
 	if (smc_cdc_before(ntohs(cdc->seqno),
 			   conn->local_rx_ctrl.seqno))
 		/* received seqno is old */
-		return;
+		goto out;
 
 	smc_cdc_msg_recv(smc, cdc);
+out:
+	sock_put(&smc->sk);
 }
 
 static struct smc_wr_rx_handler smc_cdc_rx_handlers[] = {
-- 
2.43.0