Re: [PATCH net v4] net/smc: fix lgr/lnk lifetime vs diag reader race

[Dust Li <dust.li@linux.alibaba.com>]

On 2026-09-16 14:04:42, Mahanta Jambigi wrote:
>
>
>On 15/09/26 8:43 pm, Dust Li wrote:
>> On 2026-09-11 11:09:06, Mahanta Jambigi wrote:
>>> The diag dump walks the socket hash table under a read_lock and
>>> dereferences conn->lgr and conn->lnk.  Two terminal teardown paths
>>> drop those references via smc_conn_free() while the socket is still
>>> hashed:
>>> 
>>> - smc_conn_kill() -> smc_close_active_abort() -> smc_conn_free() -
>>> smc_close_passive_work() -> smc_conn_free()
>>> 
>>> This allows the diag reader to dereference a freed lgr or lnk.
>>> 
>>> Fix it by unhashing the socket before smc_conn_free() is called at
>>> each of these two sites.  Any socket visible to the diag reader
>>> under the hash read_lock then has valid conn->lgr and conn->lnk
>>> pointers.
>>> 
>>> Fixes: f16a7dd5cf27 ("smc: netlink interface for SMC sockets") Fixes:
>>> 9dbe086c69b8 ("net/smc: fix invalid link access in dumping SMC-R
>>> connections") Signed-off-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
>>> --- Changes in v4: - dropped smc_conn_unhash() wrapper, conn-
>>> >unhashed flag, and all changes to af_smc.c, smc.h, smc_core.c and
>>> smc_core.h; smc_unhash_sk() is already idempotent via sk_hashed(),
>>> so direct calls at the two teardown sites in smc_close.c are
>>> sufficient - dropped the __smc_release() hunk: it needs no change
>>> since the subsequent unhash there is already a safe no-op - fixed
>>> premature-unhash issue present in v3: smc_conn_free() must not unhash
>>> because smc_conn_abort() calls it before smc_switch_to_fallback() in
>>> both smc_listen_decline() and smc_connect_rdma() error paths; unhashing
>>> there would make live fallback sockets invisible to smcss -
>>> likewise, the ISM/RDMA retry loops (smc_find_ism_v2_device_serv(), 
>>> smc_find_rdma_v2_device_serv()) call smc_conn_abort() on a failed attempt
>>> and then smc_conn_create() on the next device; unhashing in 
>>> smc_conn_free() would permanently hide the established connection
>>> from smc_diag since smc_conn_create() does not re-hash the socket
>> 
>> Hi Mahanta,
>> 
>> This version looks clean. And you explained why we can't call unhash
>> in smc_conn_abort() well. But smc_conn_abort() still calls
>> smc_conn_free(), when the smc_sk is still hashed, is there still a
>> race window with dump ?
>
>Hi Dust,
>
>Thank you for catching this corner case!
>
>During early handshake setup (when sk_state is *SMC_INIT*),
>smc_conn_abort() can be called on connection failure/fallback and
>invokes smc_conn_free() while the socket remains hashed, leaving a
>window where a concurrent diag dump could evaluate smc_conn_lgr_valid()
>and dereference conn->lgr / conn->lnk.
>
>Since sockets in *SMC_INIT* are in a transient embryonic handshake phase
>and userspace (smcss) *skips displaying link-group, DMB, and connection
>details for INIT state sockets anyway*, we could have __smc_diag_dump()
>skip inspecting connection/link-group extensions when r->diag_state ==
>SMC_INIT:
>
>diff --git a/net/smc/smc_diag.c b/net/smc/smc_diag.c
>--- a/net/smc/smc_diag.c
>+++ b/net/smc/smc_diag.c
>@@ -90,6 +90,9 @@ static int __smc_diag_dump(struct sock *sk, struct
>sk_buff *skb,
> 	r->diag_state = sk->sk_state;
>+	if (r->diag_state == SMC_INIT)
>+		return 0;
>+
> 	if (smc->use_fallback)
> 		r->diag_mode = SMC_DIAG_MODE_FALLBACK_TCP;
> 	else if (smc_conn_lgr_valid(&smc->conn) && smc->conn.lgr->is_smcd)
>
>Together with unhashing before smc_conn_free() in smc_close.c for
>established and closing sockets, this cleanly closes the race window
>across all socket states without touching the hash table mechanics
>during fallback/retry.
>
>Does this approach look good to you? If you agree, I will prepare and
>submit v5 with this change. Please let me know if you have any other
>suggestions or alternative approaches, and I'll be happy to look into them.

What about this path ?

smc_listen_decline() -> smc_listen_out_err(), where smc_conn_abort() has
already run, and the sock state is then changed from SMC_INIT to SMC_CLOSED.

Best regards,
Dust