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patch "USB: cdc-acm: fix rounding error in TIOCSSERIAL" added to usb-linus

PATCH [email protected] 1 participant 1 message Mar 17, 2020

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This is a note to let you know that I've just added the patch titled

USB: cdc-acm: fix rounding error in TIOCSSERIAL

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From b401f8c4f492cbf74f3f59c9141e5be3071071bb Mon Sep 17 00:00:00 2001
From: Anthony Mallet <[email protected]>
Date: Thu, 12 Mar 2020 14:31:01 +0100
Subject: 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 <[email protected]>
Cc stable <[email protected]>
Link https://lore.kernel.org/r/[email protected] open here
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 drivers/usb/class/cdc-acm.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)
drivers/usb/class/cdc-acm.c +16 -9
index da619176deca..47f09a6ce7bd 100644
@@ -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;
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