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

[Mahanta Jambigi <mjambigi@linux.ibm.com>]

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

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 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c
index bb0313ef5f7c..9e4b4e9b9d08 100644
--- a/net/smc/smc_close.c
+++ b/net/smc/smc_close.c
@@ -128,6 +128,11 @@ void smc_close_active_abort(struct smc_sock *smc)
 	struct sock *sk = &smc->sk;
 	bool release_clcsock = false;

+	/*
+	 * Unhash before any branch calls smc_conn_free() and drops lgr/lnk refs;
+	 * the diag reader must not see a hashed socket with freed lgr/lnk.
+	 */
+	sk->sk_prot->unhash(sk);
 	if (sk->sk_state != SMC_INIT && smc->clcsock && smc->clcsock->sk) {
 		sk->sk_err = ECONNABORTED;
 		if (smc->clcsock && smc->clcsock->sk)
@@ -432,7 +436,12 @@ static void smc_close_passive_work(struct work_struct *work)
 	if (old_state != sk->sk_state) {
 		sk->sk_state_change(sk);
 		if ((sk->sk_state == SMC_CLOSED) &&
 		    (sock_flag(sk, SOCK_DEAD) || !sk->sk_socket)) {
+			/*
+			 * Unhash before smc_conn_free() drops lgr/lnk refs so the diag
+			 * reader cannot see a hashed socket with freed lgr/lnk.
+			 */
+			sk->sk_prot->unhash(sk);
 			smc_conn_free(conn);
 			if (smc->clcsock)
 				release_clcsock = true;
-- 
2.43.7