Message-ID: <DM4PR12MB51683722CF9A553563D0C721E2139@DM4PR12MB5168.namprd12.prod.outlook.com>
Date:   Fri, 18 Mar 2022 22:29:59 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     Robin Murphy <robin.murphy@....com>,
        "joro@...tes.org" <joro@...tes.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "andreas.noever@...il.com" <andreas.noever@...il.com>,
        "michael.jamet@...el.com" <michael.jamet@...el.com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        "YehezkelShB@...il.com" <YehezkelShB@...il.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>
Subject: RE: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more
 accurate

[Public]

> Between me trying to get rid of iommu_present() and Mario wanting to
> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has
> shown
> that the iommu_dma_protection attribute is being far too optimistic.
> Even if an IOMMU might be present for some PCI segment in the system,
> that doesn't necessarily mean it provides translation for the device(s)
> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
> is tell us that memory was protected before the kernel was loaded, and
> prevent the user from disabling the intel-iommu driver entirely. While
> that lets us assume kernel integrity, what matters for actual runtime
> DMA protection is whether we trust individual devices, based on the
> "external facing" property that we expect firmware to describe for
> Thunderbolt ports.
> 
> It's proven challenging to determine the appropriate ports accurately
> given the variety of possible topologies, so while still not getting a
> perfect answer, by putting enough faith in firmware we can at least get
> a good bit closer. If we can see that any device near a Thunderbolt NHI
> has all the requisites for Kernel DMA Protection, chances are that it
> *is* a relevant port, but moreover that implies that firmware is playing
> the game overall, so we'll use that to assume that all Thunderbolt ports
> should be correctly marked and thus will end up fully protected.
> 

This approach looks generally good to me.  I do worry a little bit about older
systems that didn't set ExternalFacingPort in the FW but were previously setting
iommu_dma_protection, but I think that those could be treated on a quirk
basis to set PCI IDs for those root ports as external facing if/when they come
up.

I'll send up a follow up patch that adds the AMD ACPI table check.
If it looks good can roll it into your series for v3, or if this series goes
as is for v2 it can come on its own.

> CC: Mario Limonciello <mario.limonciello@....com>
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
> 
> v2: Give up trying to look for specific devices, just look for evidence
>     that firmware cares at all.

I still do think you could know exactly which devices to use if you're in
SW CM mode, but I guess the consensus is to not bifurcate the way this
can be checked.

> 
>  drivers/thunderbolt/domain.c | 12 +++--------
>  drivers/thunderbolt/nhi.c    | 41
> ++++++++++++++++++++++++++++++++++++
>  include/linux/thunderbolt.h  |  2 ++
>  3 files changed, 46 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
> index 7018d959f775..2889a214dadc 100644
> --- a/drivers/thunderbolt/domain.c
> +++ b/drivers/thunderbolt/domain.c
> @@ -7,9 +7,7 @@
>   */
> 
>  #include <linux/device.h>
> -#include <linux/dmar.h>
>  #include <linux/idr.h>
> -#include <linux/iommu.h>
>  #include <linux/module.h>
>  #include <linux/pm_runtime.h>
>  #include <linux/slab.h>
> @@ -257,13 +255,9 @@ static ssize_t iommu_dma_protection_show(struct
> device *dev,
>  					 struct device_attribute *attr,
>  					 char *buf)
>  {
> -	/*
> -	 * Kernel DMA protection is a feature where Thunderbolt security is
> -	 * handled natively using IOMMU. It is enabled when IOMMU is
> -	 * enabled and ACPI DMAR table has DMAR_PLATFORM_OPT_IN set.
> -	 */
> -	return sprintf(buf, "%d\n",
> -		       iommu_present(&pci_bus_type) &&
> dmar_platform_optin());
> +	struct tb *tb = container_of(dev, struct tb, dev);
> +
> +	return sysfs_emit(buf, "%d\n", tb->nhi->iommu_dma_protection);
>  }
>  static DEVICE_ATTR_RO(iommu_dma_protection);
> 
> diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
> index c73da0532be4..9e396e283792 100644
> --- a/drivers/thunderbolt/nhi.c
> +++ b/drivers/thunderbolt/nhi.c
> @@ -14,6 +14,7 @@
>  #include <linux/errno.h>
>  #include <linux/pci.h>
>  #include <linux/interrupt.h>
> +#include <linux/iommu.h>
>  #include <linux/module.h>
>  #include <linux/delay.h>
>  #include <linux/property.h>
> @@ -1102,6 +1103,45 @@ static void nhi_check_quirks(struct tb_nhi *nhi)
>  		nhi->quirks |= QUIRK_AUTO_CLEAR_INT;
>  }
> 
> +static int nhi_check_iommu_pdev(struct pci_dev *pdev, void *data)
> +{
> +	if (!pdev->untrusted ||
> +	    !dev_iommu_capable(&pdev->dev,
> IOMMU_CAP_PRE_BOOT_PROTECTION))
> +		return 0;
> +	*(bool *)data = true;
> +	return 1; /* Stop walking */
> +}
> +
> +static void nhi_check_iommu(struct tb_nhi *nhi)
> +{
> +	struct pci_bus *bus = nhi->pdev->bus;
> +	bool port_ok = false;
> +
> +	/*
> +	 * Ideally what we'd do here is grab every PCI device that
> +	 * represents a tunnelling adapter for this NHI and check their
> +	 * status directly, but unfortunately USB4 seems to make it
> +	 * obnoxiously difficult to reliably make any correlation.
> +	 *
> +	 * So for now we'll have to bodge it... Hoping that the system
> +	 * is at least sane enough that an adapter is in the same PCI
> +	 * segment as its NHI, if we can find *something* on that segment
> +	 * which meets the requirements for Kernel DMA Protection, we'll
> +	 * take that to imply that firmware is aware and has (hopefully)
> +	 * done the right thing in general. We need to know that the PCI
> +	 * layer has seen the ExternalFacingPort property and propagated
> +	 * it to the "untrusted" flag that the IOMMU layer will then
> +	 * enforce, but also that the IOMMU driver itself can be trusted
> +	 * not to have been subverted by a pre-boot DMA attack.
> +	 */
> +	while (bus->parent)
> +		bus = bus->parent;
> +
> +	pci_walk_bus(bus, nhi_check_iommu_pdev, &port_ok);
> +
> +	nhi->iommu_dma_protection = port_ok;
> +}
> +
>  static int nhi_init_msi(struct tb_nhi *nhi)
>  {
>  	struct pci_dev *pdev = nhi->pdev;
> @@ -1219,6 +1259,7 @@ static int nhi_probe(struct pci_dev *pdev, const
> struct pci_device_id *id)
>  		return -ENOMEM;
> 
>  	nhi_check_quirks(nhi);
> +	nhi_check_iommu(nhi);
> 
>  	res = nhi_init_msi(nhi);
>  	if (res) {
> diff --git a/include/linux/thunderbolt.h b/include/linux/thunderbolt.h
> index 124e13cb1469..7a8ad984e651 100644
> --- a/include/linux/thunderbolt.h
> +++ b/include/linux/thunderbolt.h
> @@ -465,6 +465,7 @@ static inline struct tb_xdomain
> *tb_service_parent(struct tb_service *svc)
>   * @msix_ida: Used to allocate MSI-X vectors for rings
>   * @going_away: The host controller device is about to disappear so when
>   *		this flag is set, avoid touching the hardware anymore.
> + * @iommu_dma_protection: An IOMMU will isolate external-facing ports.
>   * @interrupt_work: Work scheduled to handle ring interrupt when no
>   *		    MSI-X is used.
>   * @hop_count: Number of rings (end point hops) supported by NHI.
> @@ -479,6 +480,7 @@ struct tb_nhi {
>  	struct tb_ring **rx_rings;
>  	struct ida msix_ida;
>  	bool going_away;
> +	bool iommu_dma_protection;
>  	struct work_struct interrupt_work;
>  	u32 hop_count;
>  	unsigned long quirks;
> --
> 2.28.0.dirty

Powered by blists - more mailing lists