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: ba2d8ce9db0a6 ("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>

1 files changed, 16 insertions, 9 deletions

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 9fd1cd99aa97..13df3c8cd7bf 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -844,6 +844,7 @@ 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)))
@@ -854,18 +855,24 @@ 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 {
-		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;