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Message-ID: <8b47440b-91df-ba2f-b004-33e4c6dc1f20@arm.com>
Date:   Tue, 22 Mar 2022 14:40:02 +0000
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Subject: Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more
 accurate
Content-Language: en-GB
To:     Mika Westerberg <mika.westerberg@linux.intel.com>
Cc:     joro@8bytes.org, baolu.lu@linux.intel.com,
        andreas.noever@gmail.com, michael.jamet@intel.com,
        YehezkelShB@gmail.com, iommu@lists.linux-foundation.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        mario.limonciello@amd.com, hch@lst.de
References: <cover.1647624084.git.robin.murphy@arm.com>
 <0dd14883930c9f55ace22162e23765a37d91a057.1647624084.git.robin.murphy@arm.com>
 <Yjm150r3KPKp/2O4@lahna>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <Yjm150r3KPKp/2O4@lahna>
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On 2022-03-22 11:41, Mika Westerberg wrote:
> Hi Robin,
> 
> I tried this now on two Intel systems. One with integrated Thunderbolt
> and one with discrete. There was a small issue, see below but once fixed
> it worked as expected :)
> 
> On Fri, Mar 18, 2022 at 05:42:58PM +0000, Robin Murphy wrote:
>> 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.
>>
>> CC: Mario Limonciello <mario.limonciello@amd.com>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>
>> v2: Give up trying to look for specific devices, just look for evidence
>>      that firmware cares at all.
>>
>>   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))
> 
> This one needs to take the pdev->external_facing into account too
> because most of the time there are no existing tunnels when the driver
> is loaded so we only see the PCIe root/downstream port. I think this is
> enough actually:
> 
> 	if (!pdev->external_facing ||
> 	    !dev_iommu_capable(&pdev->dev, IOMMU_CAP_PRE_BOOT_PROTECTION))

Ah yes, my bad, for some reason I got the misapprehension into my head 
that untrusted was propagated to the port as well, not just the devices 
behind it. I'll fix this and tweak the comment below to match.

>> +		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;
> 
> I would put here a log debug, something like this:
> 
> dev_dbg(&nhi->pdev->dev, "IOMMU DMA protection is %sabled\n",
> 	port_ok ? "en" : "dis");

Ack. I'll wait and send a v3 once the merge window's over, and can roll 
Mario's AMD IOMMU patch into that too.

Thanks,
Robin.