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SubjectRe: [PATCH][serial-next] serial: 8250: don't dereference em485 until it has been null checked
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On Tue, 2017-08-29 at 17:58 +0100, Colin King wrote:

> Currently, the pointer em485 is dereferenced to get p and then later
> em485 is checked to see if it is null before calling __start_tx. In
> the case where em485 is null, we get a null pointer dereference. Fix
> this by moving the deference and the associated spinlock/unlocks on
> p to the code block where em485 is known to be not null.

>  static enum hrtimer_restart serial8250_em485_handle_start_tx(struct
> hrtimer *t)
>  {
>   struct uart_8250_em485 *em485;
> - struct uart_8250_port *p;
>   unsigned long flags;
>   em485 = container_of(t, struct uart_8250_em485,
> start_tx_timer);
> - p = em485->port;
>  
> - spin_lock_irqsave(&p->port.lock, flags);
>   if (em485 &&
>       em485->active_timer == &em485->start_tx_timer) {
> + struct uart_8250_port *p = em485->port;
> +
> + spin_lock_irqsave(&p->port.lock, flags);

Can you describe, please, what on your opinion is protected by this
lock?

>   __start_tx(&p->port);
>   em485->active_timer = NULL;
> + spin_unlock_irqrestore(&p->port.lock, flags);
>   }
> - spin_unlock_irqrestore(&p->port.lock, flags);
>   return HRTIMER_NORESTART;
>  }
>  

--
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy

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