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serial: core: Fix x_char race

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The UART driver is expected to clear port->x_char after
transmission while holding the port->lock. However, the serial
core fails to take the port->lock before assigning port->xchar.
This allows for the following race

CPU 0                         |  CPU 1
                              |
                              | serial8250_handle_irq
                              |   ...
                              |   serial8250_tx_chars
                              |     if (port->x_char)
                              |       serial_out(up, UART_TX, port->x_char)
uart_send_xchar               |
  port->x_char = ch           |
                              |       port->x_char = 0
  port->ops->start_tx()       |
                              |

The x_char on CPU 0 will never be sent.

Take the port->lock in uart_send_xchar() before assigning port->x_char.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2014-09-02 17:39:13 -04:00 • committed by Greg Kroah-Hartman
commit c235ccc1c4
1 changed files with 3 additions and 4 deletions
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7
drivers/tty/serial/serial_core.c
View file

@ -600,12 +600,11 @@ static void uart_send_xchar(struct tty_struct *tty, char ch)
if (port->ops->send_xchar)
port->ops->send_xchar(port, ch);
else {
spin_lock_irqsave(&port->lock, flags);
port->x_char = ch;
if (ch) {
spin_lock_irqsave(&port->lock, flags);
if (ch)
port->ops->start_tx(port);
spin_unlock_irqrestore(&port->lock, flags);
}
spin_unlock_irqrestore(&port->lock, flags);
}
}