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From: Greg KH <>
To: Alan Stern <>
Cc: USB list <>,,,,,,,
Subject: Re: [PATCH] USB: core: Fix races in character device registration and deregistraion
Date: Mon, 12 Aug 2019 22:52:08 +0200
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, Aug 12, 2019 at 04:11:07PM -0400, Alan Stern wrote:
> The syzbot fuzzer has found two (!) races in the USB character device
> registration and deregistration routines.  This patch fixes the races.
> The first race results from the fact that usb_deregister_dev() sets
> usb_minors[intf->minor] to NULL before calling device_destroy() on the
> class device.  This leaves a window during which another thread can
> allocate the same minor number but will encounter a duplicate name
> error when it tries to register its own class device.  A typical error
> message in the system log would look like:
>     sysfs: cannot create duplicate filename '/class/usbmisc/ldusb0'
> The patch fixes this race by destroying the class device first.
> The second race is in usb_register_dev().  When that routine runs, it
> first allocates a minor number, then drops minor_rwsem, and then
> creates the class device.  If the device creation fails, the minor
> number is deallocated and the whole routine returns an error.  But
> during the time while minor_rwsem was dropped, there is a window in
> which the minor number is allocated and so another thread can
> successfully open the device file.  Typically this results in
> use-after-free errors or invalid accesses when the other thread closes
> its open file reference, because the kernel then tries to release
> resources that were already deallocated when usb_register_dev()
> failed.  The patch fixes this race by keeping minor_rwsem locked
> throughout the entire routine.
> Reported-and-tested-by:
> Signed-off-by: Alan Stern <>
> CC: <>
> ---
> [as1907]

Thanks for this, now queued up.

greg k-h

      reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 13:18 KASAN: use-after-free Read in ld_usb_release syzbot
2019-08-09 16:51 ` Alan Stern
2019-08-09 17:33   ` Andrey Konovalov
2019-08-09 17:53     ` syzbot
2019-08-12 12:07   ` Andrey Konovalov
2019-08-12 14:21     ` Alan Stern
2019-08-12 14:31       ` Greg KH
2019-08-12 15:31   ` Greg KH
2019-08-12 20:11   ` [PATCH] USB: core: Fix races in character device registration and deregistraion Alan Stern
2019-08-12 20:52     ` Greg KH [this message]

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