TTY: serial_core, add ->install

We need to compute the uart state only on the first open. This is
usually what is done in the ->install hook. serial_core used to do this
in ->open on every open. So move it to ->install.

As a side effect, it ensures the state is set properly in the window
after tty_init_dev is called, but before uart_open. This fixes a bunch
of races between tty_open and flush_to_ldisc we were dealing with
recently.

One of such bugs was attempted to fix in commit fedb5760648a (serial:
fix race between flush_to_ldisc and tty_open), but it only took care of
a couple of functions (uart_start and uart_unthrottle). I was able to
reproduce the crash on a SLE system, but in uart_write_room which is
also called from flush_to_ldisc via process_echoes. I was *unable* to
reproduce the bug locally. It is due to having this patch in my queue
since 2012!

general protection fault: 0000 [#1] SMP KASAN PTI
CPU: 1 PID: 5 Comm: kworker/u4:0 Tainted: G L 4.12.14-396-default #1 SLE15-SP1 (unreleased)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c89-prebuilt.qemu.org 04/01/2014
Workqueue: events_unbound flush_to_ldisc
task: ffff8800427d8040 task.stack: ffff8800427f0000
RIP: 0010:uart_write_room+0xc4/0x590
RSP: 0018:ffff8800427f7088 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 000000000000002f RSI: 00000000000000ee RDI: ffff88003888bd90
RBP: ffffffffb9545850 R08: 0000000000000001 R09: 0000000000000400
R10: ffff8800427d825c R11: 000000000000006e R12: 1ffff100084fee12
R13: ffffc900004c5000 R14: ffff88003888bb28 R15: 0000000000000178
FS: 0000000000000000(0000) GS:ffff880043300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000561da0794148 CR3: 000000000ebf4000 CR4: 00000000000006e0
Call Trace:
tty_write_room+0x6d/0xc0
__process_echoes+0x55/0x870
n_tty_receive_buf_common+0x105e/0x26d0
tty_ldisc_receive_buf+0xb7/0x1c0
tty_port_default_receive_buf+0x107/0x180
flush_to_ldisc+0x35d/0x5c0
...

0 in rbx means tty->driver_data is NULL in uart_write_room. 0x178 is
tried to be dereferenced (0x178 >> 3 is 0x2f in rdx) at
uart_write_room+0xc4. 0x178 is exactly (struct uart_state *)NULL->refcount
used in uart_port_lock from uart_write_room.

So revert the upstream commit here as my local patch should fix the
whole family.

Signed-off-by: Jiri Slaby <[e-mail]>
Cc: Li RongQing <[e-mail]>
Cc: Wang Li <[e-mail]>
Cc: Zhang Yu <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/serial_core.c

···
 	struct uart_port *port;
 	unsigned long flags;
 
-	if (!state)
-		return;
-
 	port = uart_port_lock(state, flags);
 	__uart_start(tty);
 	uart_port_unlock(port, flags);
···
 	struct uart_state *state = tty->driver_data;
 	upstat_t mask = UPSTAT_SYNC_FIFO;
 	struct uart_port *port;
-
-	if (!state)
-		return;
 
 	port = uart_port_ref(state);
 	if (!port)
···
 	uart_port_deref(uport);
 }
 
+static int uart_install(struct tty_driver *driver, struct tty_struct *tty)
+{
+	struct uart_driver *drv = driver->driver_state;
+	struct uart_state *state = drv->state + tty->index;
+
+	tty->driver_data = state;
+
+	return tty_standard_install(driver, tty);
+}
+
 /*
  * Calls to uart_open are serialised by the tty_lock in
  *   drivers/tty/tty_io.c:tty_open()
···
  */
 static int uart_open(struct tty_struct *tty, struct file *filp)
 {
-	struct uart_driver *drv = tty->driver->driver_state;
-	int retval, line = tty->index;
-	struct uart_state *state = drv->state + line;
-
-	tty->driver_data = state;
+	struct uart_state *state = tty->driver_data;
+	int retval;
 
 	retval = tty_port_open(&state->port, tty, filp);
 	if (retval > 0)
···
 #endif
 
 static const struct tty_operations uart_ops = {
+	.install	= uart_install,
 	.open		= uart_open,
 	.close		= uart_close,
 	.write		= uart_write,