[PATCH 4.19 9/9] USB: core: Dont hold device lock while reading the "descriptors" sysfs file

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Alan Stern <stern@rowland.harvard.edu>

commit 45bf39f8df7f05efb83b302c65ae3b9bc92b7065 upstream.

Ever since commit 83e83ecb79a8 ("usb: core: get config and string
descriptors for unauthorized devices") was merged in 2013, there has
been no mechanism for reallocating the rawdescriptors buffers in
struct usb_device after the initial enumeration.  Before that commit,
the buffers would be deallocated when a device was deauthorized and
reallocated when it was authorized and enumerated.

This means that the locking in the read_descriptors() routine is not
needed, since the buffers it reads will never be reallocated while the
routine is running.  This locking can interfere with user programs
trying to read a hub's descriptors via sysfs while new child devices
of the hub are being initialized, since the hub is locked during this
procedure.

Since the locking in read_descriptors() hasn't been needed for over
nine years, we can remove it.

Reported-and-tested-by: Troels Liebe Bentsen <troels@connectedcars.dk>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/Y9l+wDTRbuZABzsE@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/usb/core/hub.c   |    5 ++---
 drivers/usb/core/sysfs.c |    5 -----
 2 files changed, 2 insertions(+), 8 deletions(-)

--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -2345,9 +2345,8 @@ static int usb_enumerate_device_otg(stru
  * usb_enumerate_device - Read device configs/intfs/otg (usbcore-internal)
  * @udev: newly addressed device (in ADDRESS state)
  *
- * This is only called by usb_new_device() and usb_authorize_device()
- * and FIXME -- all comments that apply to them apply here wrt to
- * environment.
+ * This is only called by usb_new_device() -- all comments that apply there
+ * apply here wrt to environment.
  *
  * If the device is WUSB and not authorized, we don't attempt to read
  * the string descriptors, as they will be errored out by the device
--- a/drivers/usb/core/sysfs.c
+++ b/drivers/usb/core/sysfs.c
@@ -888,11 +888,7 @@ read_descriptors(struct file *filp, stru
 	size_t srclen, n;
 	int cfgno;
 	void *src;
-	int retval;
 
-	retval = usb_lock_device_interruptible(udev);
-	if (retval < 0)
-		return -EINTR;
 	/* The binary attribute begins with the device descriptor.
 	 * Following that are the raw descriptor entries for all the
 	 * configurations (config plus subsidiary descriptors).
@@ -917,7 +913,6 @@ read_descriptors(struct file *filp, stru
 			off -= srclen;
 		}
 	}
-	usb_unlock_device(udev);
 	return count - nleft;
 }