Based on https://lkml.kernel.org/r/20200923092136.14824-1-oneukum@xxxxxxxx from Oliver Neukum <oneukum@xxxxxxxx>: syzbot is reporting hung task at wdm_flush() [1], for there is a circular dependency that wdm_flush() from flip_close() for /dev/cdc-wdm0 forever waits for /dev/raw-gadget to be closed while close() for /dev/raw-gadget cannot be called unless close() for /dev/cdc-wdm0 completes. Tetsuo Handa considered that such circular dependency is an usage error [2] which corresponds to an unresponding broken hardware [3]. But Alan Stern responded that we should be prepared for such hardware [4]. Therefore, this patch changes wdm_flush() to use wait_event_interruptible_timeout() which gives up after 30 seconds, for hardware that remains silent must be ignored. The 30 seconds are coming out of thin air. Changing wait_event() to wait_event_interruptible_timeout() makes error reporting from close() syscall less reliable. To compensate it, this patch also implements wdm_fsync() which does not use timeout. Those who want to be very sure that data has gone out to the device are now advised to call fsync(), with a caveat that fsync() can return -EINVAL when running on older kernels which do not implement wdm_fsync(). This patch also fixes three more problems (listed below) found during exhaustive discussion and testing. Since multiple threads can concurrently call wdm_write()/wdm_flush(), we need to use wake_up_all() whenever clearing WDM_IN_USE in order to make sure that all waiters are woken up. Also, error reporting needs to use fetch-and-clear approach in order not to report same error for multiple times. Since wdm_flush() checks WDM_DISCONNECTING, wdm_write() should as well check WDM_DISCONNECTING. In wdm_flush(), since locks are not held, it is not safe to dereference desc->intf after checking that WDM_DISCONNECTING is not set [5]. Thus, remove dev_err() from wdm_flush(). [1] https://syzkaller.appspot.com/bug?id=e7b761593b23eb50855b9ea31e3be5472b711186 [2] https://lkml.kernel.org/r/27b7545e-8f41-10b8-7c02-e35a08eb1611@xxxxxxxxxxxxxxxxxxx [3] https://lkml.kernel.org/r/79ba410f-e0ef-2465-b94f-6b9a4a82adf5@xxxxxxxxxxxxxxxxxxx [4] https://lkml.kernel.org/r/20200530011040.GB12419@xxxxxxxxxxxxxxxxxxx [5] https://lkml.kernel.org/r/c85331fc-874c-6e46-a77f-0ef1dc075308@xxxxxxxxxxxxxxxxxxx Reported-by: syzbot <syzbot+854768b99f19e89d7f81@xxxxxxxxxxxxxxxxxxxxxxxxx> Signed-off-by: Oliver Neukum <oneukum@xxxxxxxx> Debugged-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> --- drivers/usb/class/cdc-wdm.c | 72 ++++++++++++++++++++++++++++--------- 1 file changed, 55 insertions(+), 17 deletions(-) diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c index 7f5de956a2fc..02d0cfd23bb2 100644 --- a/drivers/usb/class/cdc-wdm.c +++ b/drivers/usb/class/cdc-wdm.c @@ -58,6 +58,9 @@ MODULE_DEVICE_TABLE (usb, wdm_ids); #define WDM_MAX 16 +/* we cannot wait forever at flush() */ +#define WDM_FLUSH_TIMEOUT (30 * HZ) + /* CDC-WMC r1.1 requires wMaxCommand to be "at least 256 decimal (0x100)" */ #define WDM_DEFAULT_BUFSIZE 256 @@ -151,7 +154,7 @@ static void wdm_out_callback(struct urb *urb) kfree(desc->outbuf); desc->outbuf = NULL; clear_bit(WDM_IN_USE, &desc->flags); - wake_up(&desc->wait); + wake_up_all(&desc->wait); } static void wdm_in_callback(struct urb *urb) @@ -393,6 +396,9 @@ static ssize_t wdm_write if (test_bit(WDM_RESETTING, &desc->flags)) r = -EIO; + if (test_bit(WDM_DISCONNECTING, &desc->flags)) + r = -ENODEV; + if (r < 0) { rv = r; goto out_free_mem_pm; @@ -424,6 +430,7 @@ static ssize_t wdm_write if (rv < 0) { desc->outbuf = NULL; clear_bit(WDM_IN_USE, &desc->flags); + wake_up_all(&desc->wait); /* for wdm_wait_for_response() */ dev_err(&desc->intf->dev, "Tx URB error: %d\n", rv); rv = usb_translate_errors(rv); goto out_free_mem_pm; @@ -583,28 +590,58 @@ static ssize_t wdm_read return rv; } -static int wdm_flush(struct file *file, fl_owner_t id) +static int wdm_wait_for_response(struct file *file, long timeout) { struct wdm_device *desc = file->private_data; + long rv; /* Use long here because (int) MAX_SCHEDULE_TIMEOUT < 0. */ + + /* + * Needs both flags. We cannot do with one because resetting it would + * cause a race with write() yet we need to signal a disconnect. + */ + rv = wait_event_interruptible_timeout(desc->wait, + !test_bit(WDM_IN_USE, &desc->flags) || + test_bit(WDM_DISCONNECTING, &desc->flags), + timeout); - wait_event(desc->wait, - /* - * needs both flags. We cannot do with one - * because resetting it would cause a race - * with write() yet we need to signal - * a disconnect - */ - !test_bit(WDM_IN_USE, &desc->flags) || - test_bit(WDM_DISCONNECTING, &desc->flags)); - - /* cannot dereference desc->intf if WDM_DISCONNECTING */ + /* + * To report the correct error. This is best effort. + * We are inevitably racing with the hardware. + */ if (test_bit(WDM_DISCONNECTING, &desc->flags)) return -ENODEV; - if (desc->werr < 0) - dev_err(&desc->intf->dev, "Error in flush path: %d\n", - desc->werr); + if (!rv) + return -EIO; + if (rv < 0) + return -EINTR; + + spin_lock_irq(&desc->iuspin); + rv = desc->werr; + desc->werr = 0; + spin_unlock_irq(&desc->iuspin); + + return usb_translate_errors(rv); + +} + +/* + * You need to send a signal when you react to malicious or defective hardware. + * Also, don't abort when fsync() returned -EINVAL, for older kernels which do + * not implement wdm_flush() will return -EINVAL. + */ +static int wdm_fsync(struct file *file, loff_t start, loff_t end, int datasync) +{ + return wdm_wait_for_response(file, MAX_SCHEDULE_TIMEOUT); +} - return usb_translate_errors(desc->werr); +/* + * Same with wdm_fsync(), except it uses finite timeout in order to react to + * malicious or defective hardware which ceased communication after close() was + * implicitly called due to process termination. + */ +static int wdm_flush(struct file *file, fl_owner_t id) +{ + return wdm_wait_for_response(file, WDM_FLUSH_TIMEOUT); } static __poll_t wdm_poll(struct file *file, struct poll_table_struct *wait) @@ -729,6 +766,7 @@ static const struct file_operations wdm_fops = { .owner = THIS_MODULE, .read = wdm_read, .write = wdm_write, + .fsync = wdm_fsync, .open = wdm_open, .flush = wdm_flush, .release = wdm_release, -- 2.25.1
