USB: cdc-acm: fix rounding error in TIOCSSERIAL

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>

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index da619176decac4ad658ae130015c5fa1bcece6e9..47f09a6ce7bda47a28d2b4390bce144548e49d6e 100644 (file)
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -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;