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

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Date:	Mon, 5 Jan 2015 13:19:56 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>, <x86@...nel.org>
CC:	Andrew Jones <drjones@...hat.com>, <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>, <xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Laszlo Ersek <lersek@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH] x86/xen: avoid freeing static 'name' when
 kasprintf() fails

On 05/01/15 13:06, 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.
> 
> The issue was discovered by Laszlo Ersek.

Add Reported-by: Laszlo ... tag perhaps?

> --- 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 };
>  
> @@ -420,14 +420,11 @@ 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 = ...;

>  	int irq;
>  
> @@ -438,21 +435,19 @@ void xen_setup_timer(int 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(per_cpu(xen_clock_events, cpu).name, 16, "timer%d", cpu);

Use sizeof(xevt->name) here.

David
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