linux-kernel - Re: [PATCH v2] vfio: fix noiommu vfio_iommu_group

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Date:   Wed, 30 Aug 2017 15:23:31 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio: fix noiommu vfio_iommu_group_get reference
 count

On Fri, 11 Aug 2017 15:16:06 +0200
Eric Auger <eric.auger@...hat.com> wrote:

> In vfio_iommu_group_get() we want to increase the reference
> count of the iommu group.
> 
> In noiommu case, the group does not exist and is allocated.
> iommu_group_add_device() increases the group ref count. However we
> then call iommu_group_put() which decrements it.
> 
> This leads to a "refcount_t: underflow WARN_ON".
> 
> Only decrement the ref count in case of iommu_group_add_device
> failure.
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> ---
> 
> v1 -> v2:
> put the group in case iommu_group_add_device failed
> ---
>  drivers/vfio/vfio.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

Applied to next branch for v4.14.  Thanks,

Alex
 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 330d505..4ee4f36 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -138,9 +138,10 @@ struct iommu_group *vfio_iommu_group_get(struct device *dev)
>  	iommu_group_set_name(group, "vfio-noiommu");
>  	iommu_group_set_iommudata(group, &noiommu, NULL);
>  	ret = iommu_group_add_device(group, dev);
> -	iommu_group_put(group);
> -	if (ret)
> +	if (ret) {
> +		iommu_group_put(group);
>  		return NULL;
> +	}
>  
>  	/*
>  	 * Where to taint?  At this point we've added an IOMMU group for a