[PATCH net] net/smc: fix TOCTOU race between smc_listen_out() and listener close

[Sidraya Jayagond <sidraya@linux.ibm.com>]

smc_listen_out() reads lsmc->sk.sk_state without the listener lock,
then acquires lock_sock_nested() only after the check passes. This
opens a window where smc_close_active() can transition the listener
to SMC_CLOSED, call smc_close_cleanup_listen() to drain the accept
queue, and release the lock, all between the lockless read and the
delayed lock acquisition:

  smc_listen_work (smc_hs_wq)          smc_close_active()
  -------------------------------      -------------------------
  release_sock(child)
  if (sk_state == SMC_LISTEN) TRUE
                                        lock_sock(listener)
                                        sk_state = SMC_CLOSED
                                        smc_close_cleanup_listen()
                                        release_sock(listener)
                                        flush_work(tcp_listen_work)
  lock_sock_nested(listener)
  smc_accept_enqueue(listener, child) /* child enqueued on dead listener */

smc_close_active() flushes only tcp_listen_work. Work items already
dispatched onto smc_hs_wq for the CLC handshake continue running
unguarded. smc_accept_enqueue() takes a sock_hold() on the child that
is never released, so the child smc_sock, its clcsock, and the
reference all leak. A remote peer that opens TCP connections while the
server calls close() can exhaust kernel memory.

Move lock_sock_nested() to before the sk_state check so that the test
and the enqueue are atomic under the listener lock.

Fixes: fd57770dd198 ("net/smc: wait for pending work before clcsock release_sock")
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Signed-off-by: Sidraya Jayagond <sidraya@linux.ibm.com>
---
 net/smc/af_smc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index b5db69073e20..00403175b740 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -1931,11 +1931,12 @@ static void smc_listen_out(struct smc_sock *new_smc)
 		atomic_dec(&lsmc->queued_smc_hs);
 
 	release_sock(newsmcsk); /* lock in smc_listen_work() */
+	lock_sock_nested(&lsmc->sk, SINGLE_DEPTH_NESTING);
 	if (lsmc->sk.sk_state == SMC_LISTEN) {
-		lock_sock_nested(&lsmc->sk, SINGLE_DEPTH_NESTING);
 		smc_accept_enqueue(&lsmc->sk, newsmcsk);
 		release_sock(&lsmc->sk);
 	} else { /* no longer listening */
+		release_sock(&lsmc->sk);
 		smc_close_non_accepted(newsmcsk);
 	}
 
-- 
2.53.0