From: Johan Hovold <johan@kernel.org>

commit 91fac0741d4817945c6ee0a17591421e7f5ecb86 upstream.

If the driver runs out of minor numbers it would release minor 0 and
allow another device to claim the minor while still in use.

Fortunately, registering the tty class device of the second device would
fail (with a stack dump) due to the sysfs name collision so no memory is
leaked.

Fixes: cae2bc768d17 ("usb: cdc-acm: Decrement tty port's refcount if probe() fail")
Cc: stable@vger.kernel.org      # 4.19
Cc: Jaejoong Kim <redacted>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210907082318.7757-1-johan@kernel.org (local)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/usb/class/cdc-acm.c |    7 +++++--
 drivers/usb/class/cdc-acm.h |    2 ++
 2 files changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c

@@ -726,7 +726,8 @@ static void acm_port_destruct(struct tty
 {
 	struct acm *acm = container_of(port, struct acm, port);
 
-	acm_release_minor(acm);
+	if (acm->minor != ACM_MINOR_INVALID)
+		acm_release_minor(acm);
 	usb_put_intf(acm->control);
 	kfree(acm->country_codes);
 	kfree(acm);

@@ -1343,8 +1344,10 @@ made_compressed_probe:
 	usb_get_intf(acm->control); /* undone in destruct() */
 
 	minor = acm_alloc_minor(acm);
-	if (minor < 0)
+	if (minor < 0) {
+		acm->minor = ACM_MINOR_INVALID;
 		goto alloc_fail1;
+	}
 
 	acm->minor = minor;
 	acm->dev = usb_dev;
--- a/drivers/usb/class/cdc-acm.h
+++ b/drivers/usb/class/cdc-acm.h

@@ -22,6 +22,8 @@
 #define ACM_TTY_MAJOR		166
 #define ACM_TTY_MINORS		256
 
+#define ACM_MINOR_INVALID	ACM_TTY_MINORS
+
 /*
  * Requests.
  */