FAILED: patch "[PATCH] net/smc: fix socket refcount leak in smc_switch_conns()" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 719296c4aa8213d4ac8002e77d5956d436bc98d0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090344-afraid-primarily-b3a0@gregkh' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 719296c4aa8213d4ac8002e77d5956d436bc98d0 Mon Sep 17 00:00:00 2001
From: Hidayath Khan <[email protected]>
Date: Thu, 20 Aug 2026 16:47:29 +0200
Subject: [PATCH] net/smc: fix socket refcount leak in smc_switch_conns()

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.

index 181647982490..04aedd957543 100644
@@ -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;

From: Hidayath Khan <[email protected]>

[ 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.

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

index 995c48388885c..c138a0ab0369d 100644
@@ -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