[PATCH 5.10.y] net/smc: fix socket refcount leak in smc_switch_conns()

[Sasha Levin <sashal@kernel.org>]

From: Hidayath Khan <hidayath@linux.ibm.com>

[ Upstream commit 719296c4aa8213d4ac8002e77d5956d436bc98d0 ]

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>
Link: https://patch.msgid.link/20260820144729.1019399-1-hidayath@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Replaced smc_switch_link_and_count() with the existing conn->lnk = to_lnk assignment. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 995c48388885c..c138a0ab0369d 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -582,13 +582,13 @@ 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);
-		conn->lnk = 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);
+			conn->lnk = 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;
-- 
2.53.0