USB: cdc-acm: fix unthrottle races (764478f4) · Commits · 戴 / test

Fix two long-standing bugs which could potentially lead to memory corruption or leave the port throttled until it is reopened (on weakly ordered systems), respectively, when read-URB completion races with unthrottle(). First, the URB must not be marked as free before processing is complete to prevent it from being submitted by unthrottle() on another CPU. CPU 1 CPU 2 ================ ================ complete() unthrottle() process_urb(); smp_mb__before_atomic(); set_bit(i, free); if (test_and_clear_bit(i, free)) submit_urb(); Second, the URB must be marked as free before checking the throttled flag to prevent unthrottle() on another CPU from failing to observe that the URB needs to be submitted if complete() sees that the throttled flag is set. CPU 1 CPU 2 ================ ================ complete() unthrottle() set_bit(i, free); throttled = 0; smp_mb__after_atomic(); smp_mb(); if (throttled) if (test_and_clear_bit(i, free)) return; submit_urb(); Note that test_and_clear_bit() only implies barriers when the test is successful. To handle the case where the URB is still in use an explicit barrier needs to be added to unthrottle() for the second race condition. Also note that the first race was fixed by ("cdc-acm: fix race between callback and unthrottle") back in 2015, but the bug was reintroduced a year later. Fixes: ("cdc-acm: handle read pipe errors") Fixes: ("USB: cdc-acm: re-write read processing") Signed-off-by:

Johan Hovold <johan@kernel.org> Acked-by:

Oliver Neukum <oneukum@suse.com> Cc: stable <stable@vger.kernel.org> Signed-off-by:

Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

+25 −7

Original line number	Diff line number	Diff line
		@@ -470,12 +470,12 @@ static void acm_read_bulk_callback(struct urb *urb)
		struct acm *acm = rb->instance;
		unsigned long flags;
		int status = urb->status;
		bool stopped = false;
		bool stalled = false;

		dev_vdbg(&acm->data->dev, "got urb %d, len %d, status %d\n",
		rb->index, urb->actual_length, status);

		set_bit(rb->index, &acm->read_urbs_free);

		if (!acm->dev) {
		dev_dbg(&acm->data->dev, "%s - disconnected\n", __func__);
		return;
		@@ -488,15 +488,16 @@ static void acm_read_bulk_callback(struct urb *urb)
		break;
		case -EPIPE:
		set_bit(EVENT_RX_STALL, &acm->flags);
		schedule_work(&acm->work);
		return;
		stalled = true;
		break;
		case -ENOENT:
		case -ECONNRESET:
		case -ESHUTDOWN:
		dev_dbg(&acm->data->dev,
		"%s - urb shutting down with status: %d\n",
		__func__, status);
		return;
		stopped = true;
		break;
		default:
		dev_dbg(&acm->data->dev,
		"%s - nonzero urb status received: %d\n",
		@@ -505,10 +506,24 @@ static void acm_read_bulk_callback(struct urb *urb)
		}

		/*
		* Unthrottle may run on another CPU which needs to see events
		* in the same order. Submission has an implict barrier
		* Make sure URB processing is done before marking as free to avoid
		* racing with unthrottle() on another CPU. Matches the barriers
		* implied by the test_and_clear_bit() in acm_submit_read_urb().
		*/
		smp_mb__before_atomic();
		set_bit(rb->index, &acm->read_urbs_free);
		/*
		* Make sure URB is marked as free before checking the throttled flag
		* to avoid racing with unthrottle() on another CPU. Matches the
		* smp_mb() in unthrottle().
		*/
		smp_mb__after_atomic();

		if (stopped \|\| stalled) {
		if (stalled)
		schedule_work(&acm->work);
		return;
		}

		/* throttle device if requested by tty */
		spin_lock_irqsave(&acm->read_lock, flags);
		@@ -842,6 +857,9 @@ static void acm_tty_unthrottle(struct tty_struct *tty)
		acm->throttle_req = 0;
		spin_unlock_irq(&acm->read_lock);

		/* Matches the smp_mb__after_atomic() in acm_read_bulk_callback(). */
		smp_mb();

		if (was_throttled)
		acm_submit_read_urbs(acm, GFP_KERNEL);
		}

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