Re: [PATCH] network: wireless: p54u: Fix race between disconnect and firmware loading

From: Kalle Valo
Date: Tue May 28 2019 - 08:15:02 EST

Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:

> The syzbot fuzzer found a bug in the p54 USB wireless driver. The
> issue involves a race between disconnect and the firmware-loader
> callback routine, and it has several aspects.
> One big problem is that when the firmware can't be loaded, the
> callback routine tries to unbind the driver from the USB _device_ (by
> calling device_release_driver) instead of from the USB _interface_ to
> which it is actually bound (by calling usb_driver_release_interface).
> The race involves access to the private data structure. The driver's
> disconnect handler waits for a completion that is signalled by the
> firmware-loader callback routine. As soon as the completion is
> signalled, you have to assume that the private data structure may have
> been deallocated by the disconnect handler -- even if the firmware was
> loaded without errors. However, the callback routine does access the
> private data several times after that point.
> Another problem is that, in order to ensure that the USB device
> structure hasn't been freed when the callback routine runs, the driver
> takes a reference to it. This isn't good enough any more, because now
> that the callback routine calls usb_driver_release_interface, it has
> to ensure that the interface structure hasn't been freed.
> Finally, the driver takes an unnecessary reference to the USB device
> structure in the probe function and drops the reference in the
> disconnect handler. This extra reference doesn't accomplish anything,
> because the USB core already guarantees that a device structure won't
> be deallocated while a driver is still bound to any of its interfaces.
> To fix these problems, this patch makes the following changes:
> Call usb_driver_release_interface() rather than
> device_release_driver().
> Don't signal the completion until after the important
> information has been copied out of the private data structure,
> and don't refer to the private data at all thereafter.
> Lock udev (the interface's parent) before unbinding the driver
> instead of locking udev->parent.
> During the firmware loading process, take a reference to the
> USB interface instead of the USB device.
> Don't take an unnecessary reference to the device during probe
> (and then don't drop it during disconnect).
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Reported-and-tested-by: syzbot+200d4bb11b23d929335f@xxxxxxxxxxxxxxxxxxxxxxxxx
> CC: <stable@xxxxxxxxxxxxxxx>
> ---
> [as1899]
> drivers/net/wireless/intersil/p54/p54usb.c | 43 ++++++++++++-----------------
> 1 file changed, 18 insertions(+), 25 deletions(-)

The correct prefix is "p54:", but I can fix that during commit.

> Index: usb-devel/drivers/net/wireless/intersil/p54/p54usb.c
> ===================================================================
> --- usb-devel.orig/drivers/net/wireless/intersil/p54/p54usb.c
> +++ usb-devel/drivers/net/wireless/intersil/p54/p54usb.c
> @@ -33,6 +33,8 @@ MODULE_ALIAS("prism54usb");
> MODULE_FIRMWARE("isl3886usb");
> MODULE_FIRMWARE("isl3887usb");
> +static struct usb_driver p54u_driver;

How is it safe to use static variables from a wireless driver? For
example, what if there are two p54 usb devices on the host? How do we
avoid a race in that case?

Kalle Valo