Re: [Xen-devel] [PATCH v2] x86/xen: avoid freeing static 'name' when kasprintf() fails

[Laszlo Ersek <lersek@xxxxxxxxxx>]

On 01/05/15 16:27, Vitaly Kuznetsov wrote:
> In case kasprintf() fails in xen_setup_timer() we assign name to the static
> string "<timer kasprintf failed>". We, however, don't check that fact before
> issuing kfree() in xen_teardown_timer(), kernel is supposed to crash with
> 'kernel BUG at mm/slub.c:3341!'
> 
> Solve the issue by making name a fixed length string inside struct
> xen_clock_event_device. 16 bytes should be enough.
> 
> Suggested-by: Laszlo Ersek <lersek@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> Changes from v1 (David Vrabel):
> - add 'struct xen_clock_event_device *xevt' for covenience
> - sizeof(xevt->name) in snprintf() call
> - Suggested-by: Laszlo Ersek
> ---
>  arch/x86/xen/time.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index f473d26..9f743f4 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -391,7 +391,7 @@ static const struct clock_event_device *xen_clockevent =
>  
>  struct xen_clock_event_device {
>       struct clock_event_device evt;
> -     char *name;
> +     char name[16];
>  };
>  static DEFINE_PER_CPU(struct xen_clock_event_device, xen_clock_events) = { 
> .evt.irq = -1 };
>  
> @@ -420,39 +420,33 @@ void xen_teardown_timer(int cpu)
>       if (evt->irq >= 0) {
>               unbind_from_irqhandler(evt->irq, NULL);
>               evt->irq = -1;
> -             kfree(per_cpu(xen_clock_events, cpu).name);
> -             per_cpu(xen_clock_events, cpu).name = NULL;
>       }
>  }
>  
>  void xen_setup_timer(int cpu)
>  {
> -     char *name;
> -     struct clock_event_device *evt;
> +     struct xen_clock_event_device *xevt = &per_cpu(xen_clock_events, cpu);
> +     struct clock_event_device *evt = &xevt->evt;
>       int irq;
>  
> -     evt = &per_cpu(xen_clock_events, cpu).evt;
>       WARN(evt->irq >= 0, "IRQ%d for CPU%d is already allocated\n", evt->irq, 
> cpu);
>       if (evt->irq >= 0)
>               xen_teardown_timer(cpu);
>  
>       printk(KERN_INFO "installing Xen timer for CPU %d\n", cpu);
>  
> -     name = kasprintf(GFP_KERNEL, "timer%d", cpu);
> -     if (!name)
> -             name = "<timer kasprintf failed>";
> +     snprintf(xevt->name, sizeof(xevt->name), "timer%d", cpu);
>  
>       irq = bind_virq_to_irqhandler(VIRQ_TIMER, cpu, xen_timer_interrupt,
>                                     IRQF_PERCPU|IRQF_NOBALANCING|IRQF_TIMER|
>                                     IRQF_FORCE_RESUME|IRQF_EARLY_RESUME,
> -                                   name, NULL);
> +                                   xevt->name, NULL);
>       (void)xen_set_irq_priority(irq, XEN_IRQ_PRIORITY_MAX);
>  
>       memcpy(evt, xen_clockevent, sizeof(*evt));
>  
>       evt->cpumask = cpumask_of(cpu);
>       evt->irq = irq;
> -     per_cpu(xen_clock_events, cpu).name = name;
>  }
>  
>  
> 

Looks good to me (but I defer to Xen people of course :))

Reviewed-by: Laszlo Ersek <lersek@xxxxxxxxxx>

Thanks!
Laszlo

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