Potential data race in flush_to_ldisc

[Dmitry Vyukov]

Hello,

We are working on a dynamic data race detector for the Linux kernel,
KernelThreadSanitizer (ktsan):
https://github.com/google/ktsan/wiki

While booting kernel (upstream revision 21bdb584af8c) we got a report:

ThreadSanitizer: data-race in release_tty

Write of size 8 by thread T325 (K2579):
 [<ffffffff81655c43>] release_tty+0xf3/0x1c0 drivers/tty/tty_io.c:1688
 [<ffffffff816563a8>] tty_release+0x698/0x7c0 drivers/tty/tty_io.c:1920
 [<ffffffff8126154f>] __fput+0x15f/0x310 fs/file_table.c:207
 [<ffffffff8126176d>] ____fput+0x1d/0x30 fs/file_table.c:243
 [<ffffffff810b9485>] task_work_run+0x115/0x130 kernel/task_work.c:123
(discriminator 1)
 [<     inlined    >] do_notify_resume+0x73/0x80
tracehook_notify_resume include/linux/tracehook.h:190
 [<ffffffff81006da3>] do_notify_resume+0x73/0x80 arch/x86/kernel/signal.c:757
 [<ffffffff81ee25fc>] int_signal+0x12/0x17 arch/x86/entry/entry_64.S:326

Previous read of size 8 by thread T19 (K16):
 [<ffffffff816624d9>] flush_to_ldisc+0x29/0x300 drivers/tty/tty_buffer.c:472
 [<ffffffff810b1fce>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
 [<ffffffff810b2530>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
 [<ffffffff810bbbd0>] kthread+0x150/0x170 kernel/kthread.c:207
 [<ffffffff81ee281f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:526


flush_to_ldisc accesses port->itty:

static void flush_to_ldisc(struct work_struct *work)
{
   ...
    tty = port->itty;
    if (tty == NULL)
        return;
    disc = tty_ldisc_ref(tty);

while release_tty concurrently sets itty to NULL:

static void release_tty(struct tty_struct *tty, int idx)
{
    ...
    tty->port->itty = NULL;
    if (tty->link)
        tty->link->port->itty = NULL;
    cancel_work_sync(&tty->port->buf.work);
    tty_kref_put(tty->link);
    tty_kref_put(tty);
}

It seems that read of port->itty requires to be at least READ_ONCE,
because otherwise flush_to_ldisc can check that itty is not NULL, then
re-read it again and crash with NULL deref.
I don't know what is ownership and locking story here. There can be
larger issue here: either a lock is missing, or itty can be deleted
under flush_to_ldisc feet.

Please confirm that this is real but. If so please fix it.

Thank you
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