Re: [PATCH net v7] net/smc: fix lgr/lnk lifetime vs diag reader race

The diag dump walks the socket hash table under a read_lock and dereferences
conn->lgr and conn->lnk. Three paths expose a window where a hashed socket has
freed or partially freed conn/lgr/lnk state.

First, smc_conn_abort() calls smc_conn_free() during early handshake aborts
while the socket is still hashed in SMC_INIT. On the smc_lgr_cleanup_early()
path this frees the lgr synchronously without holding the hash write_lock. A
concurrent diag reader can therefore load a non-NULL conn->lgr and dereference
lgr->is_smcd after the kfree. Guard the conn->lgr->is_smcd load in
__smc_diag_dump() by skipping it when the socket is in SMC_INIT state. The same
guard also skips the CONNINFO/LGRINFO/DMBINFO blocks for SMC_CLOSED sockets,
which may briefly remain hashed after smc_conn_free() when the fd is still open;
smcss already suppresses those attributes for closed sockets, so there is no
observability regression.

Second, in smc_conn_kill() on SMC-D with dmb_nocopy, smcd_buf_detach() NULLs and
frees conn->sndbuf_desc before the socket is unhashed. A concurrent diag reader
entering the CONNINFO block can dereference the freed descriptor. Unhash the
socket at the top of smc_conn_kill(), before smcd_buf_detach().

Third, in the terminal teardown branches of smc_close_active_abort()
(PEERCLOSEWAIT and PROCESSABORT groups) and in smc_close_passive_work(),
smc_conn_free() drops lgr and lnk references while the socket is still hashed.
Unhash immediately before each smc_conn_free() call at those two sites.

Fixes: f16a7dd5cf27 ("smc: netlink interface for SMC sockets")
Fixes: 9dbe086c69b8 ("net/smc: fix invalid link access in dumping SMC-R connections")
Fixes: ae2be35cbed2 ("net/smc: {at|de}tach sndbuf to peer DMB if supported")
Cc: stable@vger.kernel.org
Signed-off-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
---
Changes in v7:
- no code changes; commit message only
- document the SMC_CLOSED leg of the early-exit guard and the rationale
  that smcss already skips CLOSED sockets on the userspace side; add
  Fixes: ae2be35cbed2 for the smc_conn_kill() hunk since
  smcd_buf_detach() was introduced by that commit

Changes in v6:
- smc_diag.c: inline sk->sk_state != SMC_INIT into the else-if condition
  so conn->lgr->is_smcd is never loaded for SMC_INIT sockets; retain the
  SMC_INIT || SMC_CLOSED early-exit guard and out: label from v5 to
  defensively skip the CONNINFO/LGRINFO/DMBINFO blocks for sockets with
  no live connection
- smc_close.c: remove the unconditional unhash from the top of
  smc_close_active_abort(); place it scoped immediately before each of
  the two smc_conn_free() calls in the PEERCLOSEWAIT and PROCESSABORT
  branches, and before smc_conn_free() in smc_close_passive_work();
  this avoids prematurely hiding live SMC_ACTIVE/APPCLOSEWAIT sockets
  from smcss
- smc_core.c: unhash at the top of smc_conn_kill(), before
  smcd_buf_detach(), so the socket is off the hash table before
  conn->sndbuf_desc is freed on the dmb_nocopy path

Changes in v5:
- guard conn/lgr/lnk blocks in __smc_diag_dump() with an
  (SMC_INIT || SMC_CLOSED) state check placed after the
  SMC_DIAG_FALLBACK nla_put; SMC_INIT can coexist with fallback
  (smc_sendmsg() MSG_FASTOPEN path), so the guard must not suppress
  the fallback reason

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

Changes in v3:
- redesigned as a single patch; dropped the lgr_lnk_lock spinlock
  approach and the 2-patch split
- fix is now at the socket hash layer: introduce smc_conn_unhash() with
  a per-connection unhashed flag; smc_conn_free() unhashes before
  dropping lgr/lnk refs, so any socket visible to the diag reader under
  the hash read_lock has valid conn->lgr and conn->lnk pointers
- __smc_release() updated to call smc_conn_unhash() for non-fallback
  sockets so the flag is honoured when smc_conn_free() already ran first
- smc_diag.c needs no changes; the hash read_lock invariant is
  sufficient without any per-connection lock in the dump path
- dropped the clcsock/mutex_trylock fix as that will be addressed
  separately

Changes in v2:
- this is v2 of the 2-patch series; the earlier submission was
  mislabelled [PATCH v3] but was in fact the first version sent to the
  list
- split into a 2-patch series; patch 1/2 adds per-connection
  lgr_lnk_lock infrastructure to smc_core, patch 2/2 fixes the diag
  dump path using it
- dropped lock_sock()/release_sock() from __smc_diag_dump(); v1 held
  the socket lock across all lgr/lnk dereferences, requiring the hash
  read_lock to be dropped and re-acquired around each socket
- dropped the restart-from-head loop in smc_diag_dump_proto(); the new
  design does not drop the hash read_lock mid-walk so the hlist
  truncation concern no longer applies
- dropped refcount_inc_not_zero() socket pinning from the dump loop for
  the same reason: the hash read_lock is now held for the full walk
- added per-connection lgr_lnk_lock spinlock to struct smc_connection;
  conn->lgr and conn->lnk are NULLed under this lock in smc_conn_free()
  before borrowed references are released, establishing the invariant: a
  non-NULL conn->lgr seen under lgr_lnk_lock guarantees the lgr is alive
- added lgr_lnk_lock to smc_switch_link_and_count() to protect the
  conn->lnk pointer swap from concurrent diag readers
- replaced mutex_lock() on clcsock_release_lock in
  smc_diag_msg_common_fill() with mutex_trylock(); mutex_lock() was
  valid in v1 because the hash spinlock had been dropped, but the new
  design holds the hash read_lock throughout so only a non-sleeping
  trylock is safe; a failed trylock leaves address fields zeroed, which
  is acceptable for a monitoring tool
- all conn->lgr and conn->lnk accesses in __smc_diag_dump() use a
  snapshot-then-use pattern: fields are copied into local stack
  variables under lgr_lnk_lock and nla_put() is called after releasing
  the lock, avoiding any sleeping operation under the spinlock

 net/smc/smc_close.c | 3 +++
 net/smc/smc_core.c  | 1 +
 net/smc/smc_diag.c  | 7 ++++++-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c
index bb0313ef5f7c..aa6363abdd74 100644
--- a/net/smc/smc_close.c
+++ b/net/smc/smc_close.c

@@ -154,6 +154,7 @@ void smc_close_active_abort(struct smc_sock *smc)
 		if (sk->sk_state != SMC_PEERABORTWAIT)
 			break;
 		sk->sk_state = SMC_CLOSED;
+		sk->sk_prot->unhash(sk);
 		smc_conn_free(&smc->conn);
 		release_clcsock = true;
 		sock_put(sk); /* passive closing */

@@ -165,6 +166,7 @@ void smc_close_active_abort(struct smc_sock *smc)
 		if (sk->sk_state != SMC_PEERABORTWAIT)
 			break;
 		sk->sk_state = SMC_CLOSED;
+		sk->sk_prot->unhash(sk);
 		smc_conn_free(&smc->conn);
 		release_clcsock = true;
 		break;

@@ -433,6 +435,7 @@ static void smc_close_passive_work(struct work_struct *work)
 		sk->sk_state_change(sk);
 		if ((sk->sk_state == SMC_CLOSED) &&
 		    (sock_flag(sk, SOCK_DEAD) || !sk->sk_socket)) {
+			sk->sk_prot->unhash(sk);
 			smc_conn_free(conn);
 			if (smc->clcsock)
 				release_clcsock = true;

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index 04aedd957543..d7bd6d7f428b 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c

@@ -1517,6 +1517,7 @@ static void smc_conn_kill(struct smc_connection *conn, bool soft)
 {
 	struct smc_sock *smc = container_of(conn, struct smc_sock, conn);

+	smc->sk.sk_prot->unhash(&smc->sk);
 	if (conn->lgr->is_smcd && conn->lgr->peer_shutdown)
 		conn->local_tx_ctrl.conn_state_flags.peer_conn_abort = 1;
 	else

diff --git a/net/smc/smc_diag.c b/net/smc/smc_diag.c
index bf0beaa23bdb..b29bd34acf7b 100644
--- a/net/smc/smc_diag.c
+++ b/net/smc/smc_diag.c

@@ -90,7 +90,8 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 	r->diag_state = sk->sk_state;
 	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)
+	else if (sk->sk_state != SMC_INIT &&
+		 smc_conn_lgr_valid(&smc->conn) && smc->conn.lgr->is_smcd)
 		r->diag_mode = SMC_DIAG_MODE_SMCD;
 	else
 		r->diag_mode = SMC_DIAG_MODE_SMCR;

@@ -103,6 +104,9 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 	if (nla_put(skb, SMC_DIAG_FALLBACK, sizeof(fallback), &fallback) < 0)
 		goto errout;

+	if (sk->sk_state == SMC_INIT || sk->sk_state == SMC_CLOSED)
+		goto out;
+
 	if ((req->diag_ext & (1 << (SMC_DIAG_CONNINFO - 1))) &&
 	    smc->conn.alert_token_local) {
 		struct smc_connection *conn = &smc->conn;

@@ -185,6 +189,7 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 			goto errout;
 	}

+out:
 	nlmsg_end(skb, nlh);
 	return 0;