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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Alexey Kardashevskiy <aik@ozlabs.ru>
Subject: [PATCH 4.9 046/175] serial_core: Check for port state when tty is in error state
Date:   Mon, 28 Dec 2020 13:48:19 +0100
Message-Id: <20201228124855.485989423@linuxfoundation.org>
In-Reply-To: <20201228124853.216621466@linuxfoundation.org>
References: <20201228124853.216621466@linuxfoundation.org>
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From: Alexey Kardashevskiy <aik@ozlabs.ru>

commit 2f70e49ed860020f5abae4f7015018ebc10e1f0e upstream.

At the moment opening a serial device node (such as /dev/ttyS3)
succeeds even if there is no actual serial device behind it.
Reading/writing/ioctls fail as expected because the uart port is not
initialized (the type is PORT_UNKNOWN) and the TTY_IO_ERROR error state
bit is set fot the tty.

However setting line discipline does not have these checks
8250_port.c (8250 is the default choice made by univ8250_console_init()).
As the result of PORT_UNKNOWN, uart_port::iobase is NULL which
a platform translates onto some address accessing which produces a crash
like below.

This adds tty_port_initialized() to uart_set_ldisc() to prevent the crash.

Found by syzkaller.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Link: https://lore.kernel.org/r/20201203055834.45838-1-aik@ozlabs.ru
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/tty/serial/serial_core.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1415,6 +1415,10 @@ static void uart_set_ldisc(struct tty_st
 {
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *uport;
+	struct tty_port *port = &state->port;
+
+	if (!tty_port_initialized(port))
+		return;
 
 	mutex_lock(&state->port.mutex);
 	uport = uart_port_check(state);