[PATCH] tty: fix data race in flush_to_ldisc

[Dmitry Vyukov]

The data race is found with KernelThreadSanitizer (on rev 21bdb584af8c):

ThreadSanitizer: data-race in release_tty
Write of size 8 by thread T325 (K2579):
  release_tty+0xf3/0x1c0 drivers/tty/tty_io.c:1688
  tty_release+0x698/0x7c0 drivers/tty/tty_io.c:1920
  __fput+0x15f/0x310 fs/file_table.c:207
  ____fput+0x1d/0x30 fs/file_table.c:243
  task_work_run+0x115/0x130 kernel/task_work.c:123
  do_notify_resume+0x73/0x80
    tracehook_notify_resume include/linux/tracehook.h:190
  do_notify_resume+0x73/0x80 arch/x86/kernel/signal.c:757
  int_signal+0x12/0x17 arch/x86/entry/entry_64.S:326
Previous read of size 8 by thread T19 (K16):
  flush_to_ldisc+0x29/0x300 drivers/tty/tty_buffer.c:472
  process_one_work+0x47e/0x930 kernel/workqueue.c:2036
  worker_thread+0xb0/0x900 kernel/workqueue.c:2170
  kthread+0x150/0x170 kernel/kthread.c:207

flush_to_ldisc reads port->itty and checks that it is not NULL,
concurrently release_tty sets port->itty to NULL. It is possible
that flush_to_ldisc loads port->itty once, ensures that it is
not NULL, but then reloads it again and uses. The second load
can already return NULL, which will cause a crash.

Use READ_ONCE/WRITE_ONCE to read/update port->itty.

Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
---
 drivers/tty/tty_buffer.c | 2 +-
 drivers/tty/tty_io.c     | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 4cf263d..1f1031d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -469,7 +469,7 @@ static void flush_to_ldisc(struct work_struct *work)
 	struct tty_struct *tty;
 	struct tty_ldisc *disc;
 
-	tty = port->itty;
+	tty = READ_ONCE(port->itty);
 	if (tty == NULL)
 		return;
 
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 57fc6ee..aad47df 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1685,7 +1685,7 @@ static void release_tty(struct tty_struct *tty, int idx)
 	tty_driver_remove_tty(tty->driver, tty);
 	tty->port->itty = NULL;
 	if (tty->link)
-		tty->link->port->itty = NULL;
+		WRITE_ONCE(tty->link->port->itty, NULL);
 	cancel_work_sync(&tty->port->buf.work);
 
 	tty_kref_put(tty->link);
-- 
2.5.0.457.gab17608

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