[PATCH] tty: fix flush_to_ldisc() oops before tty_open is done

[taoyuhong]

From: Yuhong Tao <taoyuhong@xxxxxxxxxx>

When tty_open() is opening a serial tty at the first time, after
alloc_tty_struct() is called, before tty->ops->open() is finished.
Serial driever like pl011 on ARM is ready to setup kworker threads
to receive data with flush_to_ldisc(). Serial input at this time
window can trigger kernel oops.
On the other side, flush_to_ldisc() can also oops on a hung-up tty.
It is unknown why flush_to_ldisc() can happen after target tty is hung
up. But both of these 2 situations come into a same result, look like
that:

[   11.287911] [<ffff0000084ba198>] n_tty_receive_buf_common+0x58/0xa58
[   11.290161] [<ffff0000084baba8>] n_tty_receive_buf2+0x10/0x18
[   11.292162] [<ffff0000084bd0a0>] tty_ldisc_receive_buf+0x20/0x68
[   11.294318] [<ffff0000084bd68c>] flush_to_ldisc+0xd4/0xe8
[   11.296181] [<ffff0000080d6958>] process_one_work+0x128/0x2f0
[   11.298166] [<ffff0000080d6b74>] worker_thread+0x54/0x440
[   11.300006] [<ffff0000080dccac>] kthread+0xe4/0xf8
[   11.301642] [<ffff000008082e80>] ret_from_fork+0x10/0x50
[   11.303460] Code: b9009fbf f90047a0 d2844c00 8b170000 (c8dffc03)

Calltrace may have a bit different behind n_tty_receive_buf_common, that
is about accessing to uninitialized or realeased data struct.
Serial driver may has problem, but tty driver can easily handle these 2
oops problems by:

1. Skip data transfer of hung-up tty, in flush_to_ldisc()
2. Mark hungup to tty_struct created by tty_openi(), which will be cleaned
   at the end of tty_open().

This is tested with linux-4.13.9/Debian on ARM virtual machine, whose
serial chip is pl011. You have a little chance to watch this happen, when
keep input from keyboard during system start or shutdown. And it happens
100% if a msleep() is inserted  before tty->ops->open() is called in
tty_open(), after tty_struct is created by tty_init_dev().

Signed-off-by: Yuhong Tao <taoyuhong@xxxxxxxxxx>
---
 drivers/tty/pty.c      | 1 +
 drivers/tty/tty_io.c   | 3 +++
 drivers/tty/tty_port.c | 5 +++++
 3 files changed, 9 insertions(+)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a6d5164..ad5b075 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -853,6 +853,7 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 
 	tty_debug_hangup(tty, "opening (count=%d)\n", tty->count);
 
+	clear_bit(TTY_HUPPED, &tty->flags);
 	tty_unlock(tty);
 	return 0;
 err_release:
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 10c4038..f3abad0 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1313,6 +1313,9 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 
 	tty->port->itty = tty;
 
+	set_bit(TTY_HUPPED, &tty->flags);
+	barrier();
+
 	/*
 	 * Structures all installed ... call the ldisc open routines.
 	 * If we fail here just call release_tty to clean up.  No need
diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 6b13719..e43482f 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -34,6 +34,11 @@ static int tty_port_default_receive_buf(struct tty_port *port,
 	if (!disc)
 		return 0;
 
+	if (test_bit(TTY_HUPPED, &tty->flags)) {
+		tty_ldisc_deref(disc);
+		return 0;
+	}
+
 	ret = tty_ldisc_receive_buf(disc, p, (char *)f, count);
 
 	tty_ldisc_deref(disc);
-- 
1.8.3.1