Re: [PATCH v4 6/9] iommu/vt-d: Check whether device requires bounce buffer

Konrad Rzeszutek Wilk
2019-06-10T16:07:57Z

On Mon, Jun 03, 2019 at 09:16:17AM +0800, Lu Baolu wrote:
> This adds a helper to check whether a device needs to
> use bounce buffer. It also provides a boot time option
> to disable the bounce buffer. Users can use this to
> prevent the iommu driver from using the bounce buffer
> for performance gain.
>
> Cc: Ashok Raj <ashok.raj@intel.com>
> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Cc: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> Tested-by: Xu Pengfei <pengfei.xu@intel.com>
> Tested-by: Mika Westerberg <mika.westerberg@intel.com>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 5 +++++
> drivers/iommu/intel-iommu.c | 6 ++++++
> 2 files changed, 11 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 138f6664b2e2..65685c6e53e4 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1728,6 +1728,11 @@
> Note that using this option lowers the security
> provided by tboot because it makes the system
> vulnerable to DMA attacks.
> + nobounce [Default off]
> + Do not use the bounce buffer for untrusted devices like
> + the Thunderbolt devices. This will treat the untrusted

My brain has sometimes a hard time parsing 'Not' and 'un'. Could this be:

Disable bounce buffer for unstrusted devices ..?

And perhaps call it 'noswiotlb' ? Not everyone knows that SWIOTLB = bounce buffer.

> + devices as the trusted ones, hence might expose security
> + risks of DMA attacks.
>
> intel_idle.max_cstate= [KNL,HW,ACPI,X86]
> 0 disables intel_idle and fall back on acpi_idle.
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 235837c50719..41439647f75d 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -371,6 +371,7 @@ static int dmar_forcedac;
> static int intel_iommu_strict;
> static int intel_iommu_superpage = 1;
> static int iommu_identity_mapping;
> +static int intel_no_bounce;

intel_swiotlb_on = 1 ?

>
> #define IDENTMAP_ALL 1
> #define IDENTMAP_GFX 2
> @@ -384,6 +385,8 @@ EXPORT_SYMBOL_GPL(intel_iommu_gfx_mapped);
> static DEFINE_SPINLOCK(device_domain_lock);
> static LIST_HEAD(device_domain_list);
>
> +#define device_needs_bounce(d) (!intel_no_bounce && dev_is_untrusted(d))
> +
> /*
> * Iterate over elements in device_domain_list and call the specified
> * callback @fn against each element.
> @@ -466,6 +469,9 @@ static int __init intel_iommu_setup(char *str)
> printk(KERN_INFO
> "Intel-IOMMU: not forcing on after tboot. This could expose security risk for tboot\n");
> intel_iommu_tboot_noforce = 1;
> + } else if (!strncmp(str, "nobounce", 8)) {
> + pr_info("Intel-IOMMU: No bounce buffer. This could expose security risks of DMA attacks\n");

Again, Intel-IOMMU: No SWIOTLB. T.. blah blah'

Asking for this as doing 'dmesg | grep SWIOTLB' will expose nicely all
the SWIOTLB invocations..

> + intel_no_bounce = 1;
> }
>
> str += strcspn(str, ",");
> --
> 2.17.1
>