[PATCH net v2] net/smc: fix socket refcount leak in smc_switch_conns()

[PATCH net v2] net/smc: fix socket refcount leak in smc_switch_conns()

[Hidayath Khan]

smc_switch_conns() takes a reference on the SMC socket before dropping
lgr->conns_lock, so the connection stays alive while the CDC slot is
fetched:

        sock_hold(&smc->sk);
        read_unlock_bh(&lgr->conns_lock);
        /* pre-fetch buffer outside of send_lock, might sleep */
        rc = smc_cdc_get_free_slot(conn, to_lnk, &wr_buf, NULL, &pend);
        if (rc)
                goto err_out;

The err_out label only drops the wr_tx link reference, so this early exit
returns without the matching sock_put(). The second error exit is not
affected, because sock_put() has already run by then.

A leaked sk_refcnt means the smc_sock is never destroyed. Its send and
receive buffers stay allocated, and for a user socket the reference held
on the network namespace is never released, so the netns can no longer be
torn down.

smc_cdc_get_free_slot() fails when the target link goes down or when the
connection has been killed while the switch is in progress. Both are
reachable during the link failover this function implements, so the leak
is triggered by the same hardware events that make smc_switch_conns() run
in the first place.

Restructure so there is a single sock_put() covering both outcomes,
instead of adding a second one to the error path.

Fixes: 95f7f3e7dc6b ("net/smc: improved fix wait on already cleared link")
Cc: stable@vger.kernel.org
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Hidayath Khan <hidayath@linux.ibm.com>
---
v2:
- Restructure to a single sock_put() on the common path rather than
  adding one to the error path (Breno Leitao).

 net/smc/smc_core.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index b4208cb186c5..2f7f9d469272 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -1148,13 +1148,13 @@ struct smc_link *smc_switch_conns(struct smc_link_group *lgr,
 		read_unlock_bh(&lgr->conns_lock);
 		/* pre-fetch buffer outside of send_lock, might sleep */
 		rc = smc_cdc_get_free_slot(conn, to_lnk, &wr_buf, NULL, &pend);
-		if (rc)
-			goto err_out;
-		/* avoid race with smcr_tx_sndbuf_nonempty() */
-		spin_lock_bh(&conn->send_lock);
-		smc_switch_link_and_count(conn, to_lnk);
-		rc = smc_switch_cursor(smc, pend, wr_buf);
-		spin_unlock_bh(&conn->send_lock);
+		if (!rc) {
+			/* avoid race with smcr_tx_sndbuf_nonempty() */
+			spin_lock_bh(&conn->send_lock);
+			smc_switch_link_and_count(conn, to_lnk);
+			rc = smc_switch_cursor(smc, pend, wr_buf);
+			spin_unlock_bh(&conn->send_lock);
+		}
 		sock_put(&smc->sk);
 		if (rc)
 			goto err_out;

base-commit: a13dc7dafebe1643676c3d995f0a5651c08387c6
-- 
2.52.0