[PATCH] serial: core: fix TIOCSSERIAL and TIOCSERCONFIG on uninitialised ports

[Hengyu Liang <hengyul@cs.unc.edu>]

Commit 8ca59f7baee7 ("serial: fix ioctl hangup race") made
uart_set_info() and uart_do_autoconfig() return -EIO when TTY_IO_ERROR
is set, so that they do not run on a port that has been hung up.

However, TTY_IO_ERROR is also set on a port that could not be started,
for example because its type is unknown, and by uart_shutdown(), which
both functions call. As of now, setserial cannot configure a port that
the kernel did not detect, and an autoconfig that follows a port, irq or
type change in the same setserial call fails on any port.

The issue can be reproduced with setserial (busybox) on a port without
a detected UART, for example ttyS3 of a QEMU guest:

    setserial /dev/ttyS3 uart 16550A
    setserial -g /dev/ttyS3

Before commit 8ca59f7baee7 ("serial: fix ioctl hangup race"), the
result is:

    /dev/ttyS3, UART: 16550A, Port: 0x02e8, IRQ: 3

After that commit, the result is:

    setserial: can't set serial info: Input/output error
    /dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

This patch will check tty_port_active() in these two functions instead.
uart_hangup() clears it under the port mutex, and it stays set on an
open port that could not be started.

Fixes: 8ca59f7baee7 ("serial: fix ioctl hangup race")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
---
Commit 8ca59f7baee7 is in the stable queue for 7.2.

Tested on v7.3-rc6 in QEMU (8250). A TIOCSSERIAL that is held in
copy_from_user() with userfaultfd while the tty is hung up with
TIOCVHANGUP still gets -EIO from uart_set_info() with this patch.

 drivers/tty/serial/serial_core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 6332ed545c89..81a985424694 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -896,7 +896,11 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 	upf_t old_flags, new_flags;
 	int retval;
 
-	if (!uport || tty_io_error(tty))
+	/*
+	 * TTY_IO_ERROR is also set on a port that could not be started, and
+	 * this is how such a port is configured. Only refuse a hung up port.
+	 */
+	if (!uport || !tty_port_active(port))
 		return -EIO;
 
 	new_port = new_info->port;
@@ -1144,7 +1148,8 @@ static int uart_do_autoconfig(struct tty_struct *tty, struct uart_state *state)
 	 */
 	scoped_cond_guard(mutex_intr, return -ERESTARTSYS, &port->mutex) {
 		uport = uart_port_check(state);
-		if (!uport || tty_io_error(tty))
+		/* As in uart_set_info(), only refuse a hung up port. */
+		if (!uport || !tty_port_active(port))
 			return -EIO;
 
 		if (tty_port_users(port) != 1)
-- 
2.53.0