3.16.74-rc1 review patch. If anyone has any objections, please let me know.
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From: Johan Hovold johan@kernel.org
commit 3f5edd58d040bfa4b74fb89bc02f0bc6b9cd06ab upstream.
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.
Fixes: d83b405383c9 ("USB: serial: add support for multiple read urbs") Signed-off-by: Johan Hovold johan@kernel.org Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/usb/serial/generic.c | 39 +++++++++++++++++++++++++++++------- 1 file changed, 32 insertions(+), 7 deletions(-)
--- a/drivers/usb/serial/generic.c +++ b/drivers/usb/serial/generic.c @@ -350,6 +350,7 @@ void usb_serial_generic_read_bulk_callba struct usb_serial_port *port = urb->context; unsigned char *data = urb->transfer_buffer; unsigned long flags; + bool stopped = false; int status = urb->status; int i;
@@ -357,33 +358,51 @@ void usb_serial_generic_read_bulk_callba if (urb == port->read_urbs[i]) break; } - set_bit(i, &port->read_urbs_free);
dev_dbg(&port->dev, "%s - urb %d, len %d\n", __func__, i, urb->actual_length); switch (status) { case 0: + usb_serial_debug_data(&port->dev, __func__, urb->actual_length, + data); + port->serial->type->process_read_urb(urb); break; case -ENOENT: case -ECONNRESET: case -ESHUTDOWN: dev_dbg(&port->dev, "%s - urb stopped: %d\n", __func__, status); - return; + stopped = true; + break; case -EPIPE: dev_err(&port->dev, "%s - urb stopped: %d\n", __func__, status); - return; + stopped = true; + break; default: dev_dbg(&port->dev, "%s - nonzero urb status: %d\n", __func__, status); - goto resubmit; + break; }
- usb_serial_debug_data(&port->dev, __func__, urb->actual_length, data); - port->serial->type->process_read_urb(urb); + /* + * 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 + * usb_serial_generic_submit_read_urb(). + */ + smp_mb__before_atomic(); + set_bit(i, &port->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) + return;
-resubmit: /* Throttle the device if requested by tty */ spin_lock_irqsave(&port->lock, flags); port->throttled = port->throttle_req; @@ -458,6 +477,12 @@ void usb_serial_generic_unthrottle(struc port->throttled = port->throttle_req = 0; spin_unlock_irq(&port->lock);
+ /* + * Matches the smp_mb__after_atomic() in + * usb_serial_generic_read_bulk_callback(). + */ + smp_mb(); + if (was_throttled) usb_serial_generic_submit_read_urbs(port, GFP_KERNEL); }