This is a note to let you know that I've just added the patch titled

USB: cdc-wdm: Make wdm_flush() interruptible and add wdm_fsync().

to my usb git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git in the usb-testing branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will be merged to the usb-next branch sometime soon, after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.

From 37d2a36394d954413a495da61da1b2a51ecd28ab Mon Sep 17 00:00:00 2001

From: Oliver Neukum oneukum@suse.com Date: Mon, 28 Sep 2020 23:17:55 +0900 Subject: USB: cdc-wdm: Make wdm_flush() interruptible and add wdm_fsync().

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=e7b761593b23eb50855b9ea31e3be5472b71118... [2] https://lkml.kernel.org/r/27b7545e-8f41-10b8-7c02-e35a08eb1611@i-love.sakura... [3] https://lkml.kernel.org/r/79ba410f-e0ef-2465-b94f-6b9a4a82adf5@i-love.sakura... [4] https://lkml.kernel.org/r/20200530011040.GB12419@rowland.harvard.edu [5] https://lkml.kernel.org/r/c85331fc-874c-6e46-a77f-0ef1dc075308@i-love.sakura...

Reported-by: syzbot syzbot+854768b99f19e89d7f81@syzkaller.appspotmail.com Cc: stable stable@vger.kernel.org Co-developed-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Signed-off-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Signed-off-by: Oliver Neukum oneukum@suse.com Cc: Alan Stern stern@rowland.harvard.edu Link: https://lore.kernel.org/r/20200928141755.3476-1-penguin-kernel@I-love.SAKURA... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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,