linux-kernel - Re: [PATCH] tty: serial: owl: Fix data race in owl_uart

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Date:   Thu, 24 Jun 2021 14:29:44 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Saubhik Mukherjee <saubhik.mukherjee@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org, andrianov@...ras.ru
Subject: Re: [PATCH] tty: serial: owl: Fix data race in owl_uart_remove

On Thu, Jun 24, 2021 at 05:37:38PM +0530, Saubhik Mukherjee wrote:
> On 6/23/21 12:46 PM, Johan Hovold wrote:
> > On Wed, Jun 23, 2021 at 11:06:53AM +0530, Saubhik Mukherjee wrote:
> >> On 6/17/21 4:52 PM, Greg KH wrote:
> >>> On Thu, Jun 17, 2021 at 04:34:43PM +0530, Saubhik Mukherjee wrote:
> >>>> Suppose the driver is registered and a UART port is added. Once an
> >>>> application opens the port, owl_uart_startup is called which registers
> >>>> the interrupt handler owl_uart_irq.
> >>>>
> >>>> We could have the following race condition:
> >>>>
> >>>> When device is removed, owl_uart_remove is called, which calls
> >>>> uart_remove_one_port, which calls owl_uart_release_port, which writes
> >>>> NULL to port->membase. At this point parallely, an interrupt could be
> >>>> handled by owl_uart_irq which reads port->membase.
> >>>>
> >>>> This is because it is possible to remove device without closing a port.
> >>>> Thus, we need to check it and call owl_uart_shutdown in owl_uart_remove.
> > 
> > No, this makes no sense at all. The port is deregistered and hung up by
> > uart_remove_one_port() (and the interrupt line is consequently disabled
> > by the driver) before it is released so this can never happen.
> 
> Thanks for the reply. I am not sure I understand. I could not find any 
> interrupt disabling in owl_uart_remove. Could you point out where/how is 
> the interrupt line is disabled before releasing the port?

The interrupt line is disabled by owl_uart_shutdown(), which is called
when uart_remove_one_port() hangs up an open tty. And as I mentioned
this happens after deregistering the port (so no new opens) and before
releasing the port.

Johan