Subject: Re: [PATCH 1/4] PCI/ATS: Only enable ATS for trusted devices

From: Joerg Roedel <[email-masked]>
Date: Fri, 15 May 2020 13:57:58 +0200

Hi Jean-Philippe,

thanks for doing this!

On Fri, May 15, 2020 at 12:43:59PM +0200, Jean-Philippe Brucker wrote:
> Add pci_ats_supported(), which checks whether a device has an ATS
> capability, and whether it is trusted.  A device is untrusted if it is
> plugged into an external-facing port such as Thunderbolt and could be
> spoof an existing device to exploit weaknesses in the IOMMU
> configuration.  PCIe ATS is one such weaknesses since it allows
> endpoints to cache IOMMU translations and emit transactions with
> 'Translated' Address Type (10b) that partially bypass the IOMMU
> translation.
> 
> The SMMUv3 and VT-d IOMMU drivers already disallow ATS and transactions
> with 'Translated' Address Type for untrusted devices.  Add the check to
> pci_enable_ats() to let other drivers (AMD IOMMU for now) benefit from
> it.
> 
> By checking ats_cap, the pci_ats_supported() helper also returns whether
> ATS was globally disabled with pci=noats, and could later include more
> things, for example whether the whole PCIe hierarchy down to the
> endpoint supports ATS.
> 
> Signed-off-by: Jean-Philippe Brucker <[email-masked]>
> ---
>  include/linux/pci-ats.h |  3 +++
>  drivers/pci/ats.c       | 18 +++++++++++++++++-
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/pci-ats.h b/include/linux/pci-ats.h
> index d08f0869f1213e..f75c307f346de9 100644
> --- a/include/linux/pci-ats.h
> +++ b/include/linux/pci-ats.h
> @@ -6,11 +6,14 @@
>  
>  #ifdef CONFIG_PCI_ATS
>  /* Address Translation Service */
> +bool pci_ats_supported(struct pci_dev *dev);
>  int pci_enable_ats(struct pci_dev *dev, int ps);
>  void pci_disable_ats(struct pci_dev *dev);
>  int pci_ats_queue_depth(struct pci_dev *dev);
>  int pci_ats_page_aligned(struct pci_dev *dev);
>  #else /* CONFIG_PCI_ATS */
> +static inline bool pci_ats_supported(struct pci_dev *d)
> +{ return false; }
>  static inline int pci_enable_ats(struct pci_dev *d, int ps)
>  { return -ENODEV; }
>  static inline void pci_disable_ats(struct pci_dev *d) { }
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 390e92f2d8d1fc..15fa0c37fd8e44 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -30,6 +30,22 @@ void pci_ats_init(struct pci_dev *dev)
>  	dev->ats_cap = pos;
>  }
>  
> +/**
> + * pci_ats_supported - check if the device can use ATS
> + * @dev: the PCI device
> + *
> + * Returns true if the device supports ATS and is allowed to use it, false
> + * otherwise.
> + */
> +bool pci_ats_supported(struct pci_dev *dev)
> +{
> +	if (!dev->ats_cap)
> +		return false;
> +
> +	return !dev->untrusted;

dev->untrusted is an 'unsigned int :1', so while this works I would
prefer 'return (dev->untrusted == 0);' here, to be more type-safe.

With that changed:

Reviewed-by: Joerg Roedel <[email-masked]>

> +}
> +EXPORT_SYMBOL_GPL(pci_ats_supported);
> +
>  /**
>   * pci_enable_ats - enable the ATS capability
>   * @dev: the PCI device
> @@ -42,7 +58,7 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
>  	u16 ctrl;
>  	struct pci_dev *pdev;
>  
> -	if (!dev->ats_cap)
> +	if (!pci_ats_supported(dev))
>  		return -EINVAL;
>  
>  	if (WARN_ON(dev->ats_enabled))
> -- 
> 2.26.2

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