Re: [PATCH] xen/events: avoid NULL pointer dereference in evtchn_from_irq()

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 3/19/20 3:14 AM, Juergen Gross wrote:
> There have been reports of races in evtchn_from_irq() where the info
> pointer has been NULL.
>
> Avoid that case by testing info before dereferencing it.
>
> In order to avoid accessing a just freed info structure do the kfree()
> via kfree_rcu().


Looks like noone ever responded to this.


This change looks fine but is there a background on the problem? I
looked in the archives and didn't find the relevant discussion.


-boris


>
> Cc: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Cc: stable@vger.kernel.org
> Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  drivers/xen/events/events_base.c     | 10 ++++++++--
>  drivers/xen/events/events_internal.h |  3 +++
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> index 499eff7d3f65..838762fe3d6e 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -247,10 +247,16 @@ static void xen_irq_info_cleanup(struct irq_info *info)
>   */
>  unsigned int evtchn_from_irq(unsigned irq)
>  {
> +	struct irq_info *info;
> +
>  	if (WARN(irq >= nr_irqs, "Invalid irq %d!\n", irq))
>  		return 0;
>  
> -	return info_for_irq(irq)->evtchn;
> +	info = info_for_irq(irq);
> +	if (info == NULL)
> +		return 0;
> +
> +	return info->evtchn;
>  }
>  
>  unsigned irq_from_evtchn(unsigned int evtchn)
> @@ -436,7 +442,7 @@ static void xen_free_irq(unsigned irq)
>  
>  	WARN_ON(info->refcnt > 0);
>  
> -	kfree(info);
> +	kfree_rcu(info, rcu);
>  
>  	/* Legacy IRQ descriptors are managed by the arch. */
>  	if (irq < nr_legacy_irqs())
> diff --git a/drivers/xen/events/events_internal.h b/drivers/xen/events/events_internal.h
> index 82938cff6c7a..c421055843c8 100644
> --- a/drivers/xen/events/events_internal.h
> +++ b/drivers/xen/events/events_internal.h
> @@ -7,6 +7,8 @@
>  #ifndef __EVENTS_INTERNAL_H__
>  #define __EVENTS_INTERNAL_H__
>  
> +#include <linux/rcupdate.h>
> +
>  /* Interrupt types. */
>  enum xen_irq_type {
>  	IRQT_UNBOUND = 0,
> @@ -30,6 +32,7 @@ enum xen_irq_type {
>   */
>  struct irq_info {
>  	struct list_head list;
> +	struct rcu_head rcu;
>  	int refcnt;
>  	enum xen_irq_type type;	/* type */
>  	unsigned irq;