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USB: cdc-acm: fix rounding error in TIOCSSERIAL

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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: ba2d8ce9db ("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>
This commit is contained in:
Anthony Mallet 2020-03-12 14:31:01 +01:00 committed by Greg Kroah-Hartman
parent 633e2b2ded
commit b401f8c4f4
1 changed files with 16 additions and 9 deletions
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25
drivers/usb/class/cdc-acm.c
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@ -907,6 +907,7 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)
{
struct acm *acm = tty->driver_data;
unsigned int closing_wait, close_delay;
unsigned int old_closing_wait, old_close_delay;
int retval = 0;
close_delay = msecs_to_jiffies(ss->close_delay * 10);
@ -914,18 +915,24 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)
ASYNC_CLOSING_WAIT_NONE :
msecs_to_jiffies(ss->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 ((ss->close_delay != old_close_delay) ||
(ss->closing_wait != old_closing_wait)) {
if (!capable(CAP_SYS_ADMIN))
retval = -EPERM;
else
retval = -EOPNOTSUPP;
} else {
acm->port.close_delay = close_delay;
acm->port.closing_wait = closing_wait;
}
else {
acm->port.close_delay = close_delay;
acm->port.closing_wait = closing_wait;
}
} else
retval = -EOPNOTSUPP;
mutex_unlock(&acm->port.mutex);
return retval;