[PATCH 5.4 20/54] cifs: fix potential use-after-free in cifs_echo_request()

[Greg Kroah-Hartman]

From: Paulo Alcantara <pc@xxxxxx>

commit 212253367dc7b49ed3fc194ce71b0992eacaecf2 upstream.

This patch fixes a potential use-after-free bug in
cifs_echo_request().

For instance,

  thread 1
  --------
  cifs_demultiplex_thread()
    clean_demultiplex_info()
      kfree(server)

  thread 2 (workqueue)
  --------
  apic_timer_interrupt()
    smp_apic_timer_interrupt()
      irq_exit()
        __do_softirq()
          run_timer_softirq()
            call_timer_fn()
	      cifs_echo_request() <- use-after-free in server ptr

Signed-off-by: Paulo Alcantara (SUSE) <pc@xxxxxx>
CC: Stable <stable@xxxxxxxxxxxxxxx>
Reviewed-by: Ronnie Sahlberg <lsahlber@xxxxxxxxxx>
Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/cifs/connect.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -975,6 +975,8 @@ static void clean_demultiplex_info(struc
 	list_del_init(&server->tcp_ses_list);
 	spin_unlock(&cifs_tcp_ses_lock);
 
+	cancel_delayed_work_sync(&server->echo);
+
 	spin_lock(&GlobalMid_Lock);
 	server->tcpStatus = CifsExiting;
 	spin_unlock(&GlobalMid_Lock);