From be4219dd98608736e13e0b790ef742b76a13254d Mon Sep 17 00:00:00 2001 From: Johan Hovold Date: Mon, 7 Sep 2026 11:51:30 +0200 Subject: [PATCH] USB: cdc-acm: fix racy TIOCMIWAIT implementation Check the wakeup condition after adding the task to the waitqueue and updating the task state to avoid missing a racing modem status update or disconnect. Store the old icount on entry and do not update it in the completion handler to avoid missing events or reporting spurious ones (due to modem status changes before TIOCMIWAIT was called). Fixes: 5a6a62bdb925 ("cdc-acm: add TIOCMIWAIT") Cc: stable Cc: Oliver Neukum Signed-off-by: Johan Hovold Link: https://patch.msgid.link/20260907095130.130636-1-johan@kernel.org Signed-off-by: Greg Kroah-Hartman --- drivers/usb/class/cdc-acm.c | 36 +++++++++++++++++++----------------- drivers/usb/class/cdc-acm.h | 1 - 2 files changed, 19 insertions(+), 18 deletions(-) diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c index 7bc5329fa3ed..21417b4c5154 100644 --- a/drivers/usb/class/cdc-acm.c +++ b/drivers/usb/class/cdc-acm.c @@ -331,7 +331,6 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf) spin_lock_irqsave(&acm->read_lock, flags); acm->ctrlin = newctrl; - acm->oldcount = acm->iocount; if (difference & USB_CDC_SERIAL_STATE_DSR) acm->iocount.dsr++; @@ -1027,11 +1026,16 @@ static int wait_serial_change(struct acm *acm, unsigned long arg) DECLARE_WAITQUEUE(wait, current); struct async_icount old, new; - do { + spin_lock_irq(&acm->read_lock); + old = acm->iocount; + spin_unlock_irq(&acm->read_lock); + + add_wait_queue(&acm->wioctl, &wait); + for (;;) { + set_current_state(TASK_INTERRUPTIBLE); + spin_lock_irq(&acm->read_lock); - old = acm->oldcount; new = acm->iocount; - acm->oldcount = new; spin_unlock_irq(&acm->read_lock); if ((arg & TIOCM_DSR) && @@ -1044,22 +1048,20 @@ static int wait_serial_change(struct acm *acm, unsigned long arg) old.rng != new.rng) break; - add_wait_queue(&acm->wioctl, &wait); - set_current_state(TASK_INTERRUPTIBLE); - schedule(); - remove_wait_queue(&acm->wioctl, &wait); if (acm->disconnected) { - if (arg & TIOCM_CD) - break; - else - rv = -ENODEV; - } else { - if (signal_pending(current)) - rv = -ERESTARTSYS; + rv = -ENODEV; + break; } - } while (!rv); - + schedule(); + + if (signal_pending(current)) { + rv = -ERESTARTSYS; + break; + } + } + __set_current_state(TASK_RUNNING); + remove_wait_queue(&acm->wioctl, &wait); return rv; } diff --git a/drivers/usb/class/cdc-acm.h b/drivers/usb/class/cdc-acm.h index 01f448a783c0..d245e60eb9f5 100644 --- a/drivers/usb/class/cdc-acm.h +++ b/drivers/usb/class/cdc-acm.h @@ -90,7 +90,6 @@ struct acm { unsigned int ctrlin; /* input control lines (DCD, DSR, RI, break, overruns) */ unsigned int ctrlout; /* output control lines (DTR, RTS) */ struct async_icount iocount; /* counters for control line changes */ - struct async_icount oldcount; /* for comparison of counter */ wait_queue_head_t wioctl; /* for ioctl */ unsigned int writesize; /* max packet size for the output bulk endpoint */ unsigned int readsize,ctrlsize; /* buffer sizes for freeing */