[PATCH net v1] net/smc: fix socket use-after-free during link group termination

[PATCH net v1] net/smc: fix socket use-after-free during link group termination

[xuanqiang.luo]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

__smc_lgr_terminate() drops conns_lock after finding a connection in
lgr->conns_all, but before taking a reference on its socket. The connection
is embedded in the socket, and its registration reference protects it only
while the connection remains in the tree.

A concurrent close can unregister the connection and drop that reference,
freeing the socket before the termination worker reaches sock_hold().

The race is reachable when close overlaps link group termination.
Local stress testing reproduced the use-after-free and KASAN reported:

  BUG: KASAN: slab-use-after-free in __smc_lgr_terminate.part.0 [smc]
  Write of size 4 by task kworker/3:3
  Workqueue: events smc_lgr_terminate_work [smc]
  __smc_lgr_terminate.part.0 [smc]

The socket was allocated by smc_create(), freed through
slab_free_after_rcu_debug(), and was followed by:

  refcount_t: addition on 0; use-after-free.
  __smc_lgr_terminate.part.0 [smc]

Take the socket reference while conns_lock still protects the tree entry.
The unregister path then cannot drop the last reference until termination
has finished using the socket.

Fixes: 69318b5215f2 ("net/smc: improve abnormal termination locking")
Cc: stable@vger.kernel.org
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
 net/smc/smc_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index cf6b620fef05f..b4208cb186c5e 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -1572,10 +1572,10 @@ static void __smc_lgr_terminate(struct smc_link_group *lgr, bool soft)
 	read_lock_bh(&lgr->conns_lock);
 	node = rb_first(&lgr->conns_all);
 	while (node) {
-		read_unlock_bh(&lgr->conns_lock);
 		conn = rb_entry(node, struct smc_connection, alert_node);
 		smc = container_of(conn, struct smc_sock, conn);
 		sock_hold(&smc->sk); /* sock_put below */
+		read_unlock_bh(&lgr->conns_lock);
 		lock_sock(&smc->sk);
 		smc_conn_kill(conn, soft);
 		release_sock(&smc->sk);
-- 
2.43.0

Re: [PATCH net v1] net/smc: fix socket use-after-free during link group termination

[Mahanta Jambigi]

On 23/07/26 4:24 pm, xuanqiang.luo@linux.dev wrote:
> From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> 
> __smc_lgr_terminate() drops conns_lock after finding a connection in
> lgr->conns_all, but before taking a reference on its socket. The connection
> is embedded in the socket, and its registration reference protects it only
> while the connection remains in the tree.
> 
> A concurrent close can unregister the connection and drop that reference,
> freeing the socket before the termination worker reaches sock_hold().
> 
> The race is reachable when close overlaps link group termination.
> Local stress testing reproduced the use-after-free and KASAN reported:
> 
>   BUG: KASAN: slab-use-after-free in __smc_lgr_terminate.part.0 [smc]
>   Write of size 4 by task kworker/3:3
>   Workqueue: events smc_lgr_terminate_work [smc]
>   __smc_lgr_terminate.part.0 [smc]
> 
> The socket was allocated by smc_create(), freed through
> slab_free_after_rcu_debug(), and was followed by:
> 
>   refcount_t: addition on 0; use-after-free.
>   __smc_lgr_terminate.part.0 [smc]
> 
> Take the socket reference while conns_lock still protects the tree entry.
> The unregister path then cannot drop the last reference until termination
> has finished using the socket.

The race description and fix look correct to me. Taking the socket
reference before dropping *conns_lock* closes the window where a
concurrent close can free the socket before sock_hold().

> 
> Fixes: 69318b5215f2 ("net/smc: improve abnormal termination locking")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>

> ---
>  net/smc/smc_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> index cf6b620fef05f..b4208cb186c5e 100644
> --- a/net/smc/smc_core.c
> +++ b/net/smc/smc_core.c
> @@ -1572,10 +1572,10 @@ static void __smc_lgr_terminate(struct smc_link_group *lgr, bool soft)
>  	read_lock_bh(&lgr->conns_lock);
>  	node = rb_first(&lgr->conns_all);
>  	while (node) {
> -		read_unlock_bh(&lgr->conns_lock);
>  		conn = rb_entry(node, struct smc_connection, alert_node);
>  		smc = container_of(conn, struct smc_sock, conn);
>  		sock_hold(&smc->sk); /* sock_put below */
> +		read_unlock_bh(&lgr->conns_lock);
>  		lock_sock(&smc->sk);
>  		smc_conn_kill(conn, soft);
>  		release_sock(&smc->sk);

Re: [PATCH net v1] net/smc: fix socket use-after-free during link group termination

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Thu, 23 Jul 2026 18:54:54 +0800 you wrote:
> From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> 
> __smc_lgr_terminate() drops conns_lock after finding a connection in
> lgr->conns_all, but before taking a reference on its socket. The connection
> is embedded in the socket, and its registration reference protects it only
> while the connection remains in the tree.
> 
> [...]

Here is the summary with links:
  - [net,v1] net/smc: fix socket use-after-free during link group termination
    https://git.kernel.org/netdev/net/c/f621d6ebeebb

You are awesome, thank you!
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