Commit 72d3dd25 authored Mar 12, 2020 by Anthony Mallet Committed by Greg Kroah-Hartman Mar 25, 2020

USB: cdc-acm: fix rounding error in TIOCSSERIAL

[ Upstream commit b401f8c4f492cbf74f3f59c9141e5be3071071bb ]

By default, tty_port_init() initializes those parameters to a multiple
of HZ. For instance in line 69 of tty_port.c:
   port->close_delay = (50 * HZ) / 100;
https://github.com/torvalds/linux/blob/master/drivers/tty/tty_port.c#L69

With e.g. CONFIG_HZ = 250 (as this is the case for Ubuntu 18.04
linux-image-4.15.0-37-generic), the default setting for close_delay is
thus 125.

When ioctl(fd, TIOCGSERIAL, &s) is executed, the setting returned in
user space is '12' (125/10). When ioctl(fd, TIOCSSERIAL, &s) is then
executed with the same setting '12', the value is interpreted as '120'
which is different from the current setting and a EPERM error may be
raised by set_serial_info() if !CAP_SYS_ADMIN.
https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L919



Fixes: ba2d8ce9 ("cdc-acm: implement TIOCSSERIAL to avoid blocking close(2)")
Signed-off-by: Anthony Mallet <anthony.mallet@laas.fr>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200312133101.7096-2-anthony.mallet@laas.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent c6261242

drivers/usb/class/cdc-acm.c

+16 −9

Original line number	Diff line number	Diff line
		@@ -930,6 +930,7 @@ static int set_serial_info(struct acm *acm,
		{
		struct serial_struct new_serial;
		unsigned int closing_wait, close_delay;
		unsigned int old_closing_wait, old_close_delay;
		int retval = 0;

		if (copy_from_user(&new_serial, newinfo, sizeof(new_serial)))
		@@ -940,18 +941,24 @@ static int set_serial_info(struct acm *acm,
		ASYNC_CLOSING_WAIT_NONE :
		msecs_to_jiffies(new_serial.closing_wait * 10);

		/* we must redo the rounding here, so that the values match */
		old_close_delay = jiffies_to_msecs(acm->port.close_delay) / 10;
		old_closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
		ASYNC_CLOSING_WAIT_NONE :
		jiffies_to_msecs(acm->port.closing_wait) / 10;

		mutex_lock(&acm->port.mutex);

		if (!capable(CAP_SYS_ADMIN)) {
		if ((close_delay != acm->port.close_delay) \|\|
		(closing_wait != acm->port.closing_wait))
		if ((new_serial.close_delay != old_close_delay) \|\|
		(new_serial.closing_wait != old_closing_wait)) {
		if (!capable(CAP_SYS_ADMIN))
		retval = -EPERM;
		else
		retval = -EOPNOTSUPP;
		} else {
		else {
		acm->port.close_delay = close_delay;
		acm->port.closing_wait = closing_wait;
		}
		} else
		retval = -EOPNOTSUPP;

		mutex_unlock(&acm->port.mutex);
		return retval;