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serial: Fix locking for uart driver set_termios() method

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The low-level uart driver may modify termios settings to override
settings that are not compatible with the uart, such as CRTSCTS.
Thus, callers of the low-level uart driver's set_termios() method must
hold termios_rwsem write lock to prevent concurrent access to termios,
in case such override occurs.

The termios_rwsem lock requirement does not extend to console setup
(ie., uart_set_options), as console setup cannot race with tty
operations. Nor does this lock requirement extend to functions which
cannot be concurrent with tty ioctls (ie., uart_port_startup() and
uart_resume_port()).

Further, always claim the port mutex to protect hardware
re-reprogramming in the set_termios() uart driver method. Note this
is unnecessary for console initialization in uart_set_options()
which cannot be concurrent with other uart operations.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2014-10-16 16:54:20 -04:00 • committed by Greg Kroah-Hartman
parent 2e75891083
commit 7c8ab967e3
2 changed files with 11 additions and 3 deletions
Documentation/serial
driver
drivers/tty/serial
serial_core.c

6
Documentation/serial/driver
View file

@ -59,7 +59,9 @@ The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
access to the info->tmpbuf bouncebuffer used for port writes. access to the info->tmpbuf bouncebuffer used for port writes.
The port_sem semaphore is used to protect against ports being added/ The port_sem semaphore is used to protect against ports being added/
removed or reconfigured at inappropriate times. removed or reconfigured at inappropriate times. Since v2.6.27, this
semaphore has been the 'mutex' member of the tty_port struct, and
commonly referred to as the port mutex (or port->mutex).
uart_ops uart_ops
@ -248,7 +250,7 @@ hardware.
Other flags may be used (eg, xon/xoff characters) if your Other flags may be used (eg, xon/xoff characters) if your
hardware supports hardware "soft" flow control. hardware supports hardware "soft" flow control.
Locking: none. Locking: caller holds port->mutex
Interrupts: caller dependent. Interrupts: caller dependent.
This call must not sleep This call must not sleep

8
drivers/tty/serial/serial_core.c
View file

@ -436,7 +436,7 @@ uart_get_divisor(struct uart_port *port, unsigned int baud)
EXPORT_SYMBOL(uart_get_divisor); EXPORT_SYMBOL(uart_get_divisor);
/* FIXME: Consistent locking policy */ /* Caller holds port mutex */
static void uart_change_speed(struct tty_struct *tty, struct uart_state *state, static void uart_change_speed(struct tty_struct *tty, struct uart_state *state,
struct ktermios *old_termios) struct ktermios *old_termios)
{ {
@ -1173,11 +1173,15 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd,
break; break;
case TIOCSSERIAL: case TIOCSSERIAL:
down_write(&tty->termios_rwsem);
ret = uart_set_info_user(tty, state, uarg); ret = uart_set_info_user(tty, state, uarg);
up_write(&tty->termios_rwsem);
break; break;
case TIOCSERCONFIG: case TIOCSERCONFIG:
down_write(&tty->termios_rwsem);
ret = uart_do_autoconfig(tty, state); ret = uart_do_autoconfig(tty, state);
up_write(&tty->termios_rwsem);
break; break;
case TIOCSERGWILD: /* obsolete */ case TIOCSERGWILD: /* obsolete */
@ -1278,7 +1282,9 @@ static void uart_set_termios(struct tty_struct *tty,
return; return;
} }
mutex_lock(&state->port.mutex);
uart_change_speed(tty, state, old_termios); uart_change_speed(tty, state, old_termios);
mutex_unlock(&state->port.mutex);
/* reload cflag from termios; port driver may have overriden flags */ /* reload cflag from termios; port driver may have overriden flags */
cflag = tty->termios.c_cflag; cflag = tty->termios.c_cflag;