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

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Date:	Mon, 05 Jan 2015 16:36:07 +0100
From:	Laszlo Ersek <lersek@...hat.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v2] x86/xen: avoid freeing static 'name' when kasprintf()
 fails

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@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> 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@...hat.com>

Thanks!
Laszlo
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