Re: [PATCH] usb: usbfs: fix use-after-free of usb_device in usbdev_release()

On Mon, Aug 10, 2026 at 02:12:09PM +0200, Miguel Peñaranda wrote:
> usbdev_release() drops its reference to the struct usb_device before
> draining the list of completed async URBs, but that drain path reads back
> through the same object: free_async() calls dec_usb_memory_use_count()
> for any URB whose buffer came from the usbfs mmap() region, and its first
> statement is bus_to_hcd(ps->dev->bus).
> 
> After a disconnect the usbfs reference can be the last one, in which case
> usb_put_dev() frees the device and the subsequent loop reads offset 80 of
> freed memory and uses the result as a struct usb_hcd *, which
> hcd_buffer_free_pages() then dereferences.
> 
> This is reachable by an unprivileged process that has read/write access to
> a /dev/bus/usb node: mmap() the fd, submit one URB with a buffer inside the
> mapping, wait for the device to be unplugged, then munmap() and close().
> It reproduces on every attempt rather than being a race, because a live
> MAP_SHARED vma holds a reference on the struct file, so usbdev_release()
> cannot run until the last vma is gone and the freeing branch of
> dec_usb_memory_use_count() is always taken.
> 
>   BUG: KASAN: slab-use-after-free in dec_usb_memory_use_count+0x3ae/0x410
>   Read of size 8 at addr ffff8880122ee050 by task poc/769
>   CPU: 1 UID: 1000 PID: 769 Comm: poc Tainted: G    B    6.12.94 #3
> 
>   Call Trace:
>    dec_usb_memory_use_count+0x3ae/0x410
>    free_async+0x2aa/0x4f0
>    usbdev_release+0x375/0x460
>    __fput+0x3ea/0xb50
>    __x64_sys_close+0x86/0x100
> 
>   Allocated by task 11:
>    usb_alloc_dev+0x55/0xd90
>    hub_event+0x2524/0x43d0
> 
>   Freed by task 769:
>    kfree+0x121/0x360
>    device_release+0xd2/0x280
>    usb_put_dev+0x23/0x30
>    usbdev_release+0x2d8/0x460
> 
> Release the device reference after the drain loop instead. Nothing between
> the two points requires it to have been dropped.
> 
> Fixes: f7d34b445abc ("USB: Add support for usbfs zerocopy.")
> Cc: [email protected]
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Miguel Peñaranda <[email protected]>
> ---

Reviewed-by: Alan Stern <[email protected]>

>  drivers/usb/core/devio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
> index e191934..486ced7 100644
> --- a/drivers/usb/core/devio.c
> +++ b/drivers/usb/core/devio.c
> @@ -1113,7 +1113,6 @@ static int usbdev_release(struct inode *inode, struct file *file)
>  	if (!ps->suspend_allowed)
>  		usb_autosuspend_device(dev);
>  	usb_unlock_device(dev);
> -	usb_put_dev(dev);
>  	put_pid(ps->disc_pid);
>  	put_cred(ps->cred);
>  
> @@ -1122,6 +1121,7 @@ static int usbdev_release(struct inode *inode, struct file *file)
>  		free_async(as);
>  		as = async_getcompleted(ps);
>  	}
> +	usb_put_dev(dev);
>  
>  	kfree(ps);
>  	return 0;
> -- 
> 2.53.0
> 
>