Re: [PATCH 2/2] xen/virtio: Fix potential deadlock when accessing xen_grant_dma_devices

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 5 Oct 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> As find_xen_grant_dma_data() is called from both interrupt and process
> contexts, the access to xen_grant_dma_devices XArray must be protected
> by xa_lock_irqsave to avoid deadlock scenario.
> As XArray API doesn't provide xa_store_irqsave helper, call lockless
> __xa_store directly and guard it externally.
> 
> Also move the storage of the XArray's entry to a separate helper.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> Fixes: d6aca3504c7d ("xen/grant-dma-ops: Add option to restrict memory access under Xen")

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
>  drivers/xen/grant-dma-ops.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> index 1998d0e8ce82..c66f56d24013 100644
> --- a/drivers/xen/grant-dma-ops.c
> +++ b/drivers/xen/grant-dma-ops.c
> @@ -25,7 +25,7 @@ struct xen_grant_dma_data {
>  	bool broken;
>  };
>  
> -static DEFINE_XARRAY(xen_grant_dma_devices);
> +static DEFINE_XARRAY_FLAGS(xen_grant_dma_devices, XA_FLAGS_LOCK_IRQ);
>  
>  #define XEN_GRANT_DMA_ADDR_OFF	(1ULL << 63)
>  
> @@ -42,14 +42,29 @@ static inline grant_ref_t dma_to_grant(dma_addr_t dma)
>  static struct xen_grant_dma_data *find_xen_grant_dma_data(struct device *dev)
>  {
>  	struct xen_grant_dma_data *data;
> +	unsigned long flags;
>  
> -	xa_lock(&xen_grant_dma_devices);
> +	xa_lock_irqsave(&xen_grant_dma_devices, flags);
>  	data = xa_load(&xen_grant_dma_devices, (unsigned long)dev);
> -	xa_unlock(&xen_grant_dma_devices);
> +	xa_unlock_irqrestore(&xen_grant_dma_devices, flags);
>  
>  	return data;
>  }
>  
> +static int store_xen_grant_dma_data(struct device *dev,
> +				    struct xen_grant_dma_data *data)
> +{
> +	unsigned long flags;
> +	int ret;
> +
> +	xa_lock_irqsave(&xen_grant_dma_devices, flags);
> +	ret = xa_err(__xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
> +			GFP_ATOMIC));
> +	xa_unlock_irqrestore(&xen_grant_dma_devices, flags);
> +
> +	return ret;
> +}
> +
>  /*
>   * DMA ops for Xen frontends (e.g. virtio).
>   *
> @@ -338,8 +353,7 @@ void xen_grant_setup_dma_ops(struct device *dev)
>  	 */
>  	data->backend_domid = iommu_spec.args[0];
>  
> -	if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
> -			GFP_KERNEL))) {
> +	if (store_xen_grant_dma_data(dev, data)) {
>  		dev_err(dev, "Cannot store Xen grant DMA data\n");
>  		goto err;
>  	}
> -- 
> 2.25.1
>