Re: [PATCH v2 12/12] iommu: Do not allow IOMMU passthrough with Secure Launch

Ross Philipson
2021-06-30T09:50:47Z

On 6/21/21 5:15 PM, Andy Lutomirski wrote:
> On Mon, Jun 21, 2021 at 10:51 AM Ross Philipson
> <ross.philipson@oracle.com> wrote:
>>
>> On 6/18/21 2:32 PM, Robin Murphy wrote:
>>> On 2021-06-18 17:12, Ross Philipson wrote:
>>>> The IOMMU should always be set to default translated type after
>>>> the PMRs are disabled to protect the MLE from DMA.
>>>>
>>>> Signed-off-by: Ross Philipson <ross.philipson@oracle.com>
>>>> ---
>>>> drivers/iommu/intel/iommu.c | 5 +++++
>>>> drivers/iommu/iommu.c | 6 +++++-
>>>> 2 files changed, 10 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>>> index be35284..4f0256d 100644
>>>> --- a/drivers/iommu/intel/iommu.c
>>>> +++ b/drivers/iommu/intel/iommu.c
>>>> @@ -41,6 +41,7 @@
>>>> #include <linux/dma-direct.h>
>>>> #include <linux/crash_dump.h>
>>>> #include <linux/numa.h>
>>>> +#include <linux/slaunch.h>
>>>> #include <asm/irq_remapping.h>
>>>> #include <asm/cacheflush.h>
>>>> #include <asm/iommu.h>
>>>> @@ -2877,6 +2878,10 @@ static bool device_is_rmrr_locked(struct device
>>>> *dev)
>>>> */
>>>> static int device_def_domain_type(struct device *dev)
>>>> {
>>>> + /* Do not allow identity domain when Secure Launch is configured */
>>>> + if (slaunch_get_flags() & SL_FLAG_ACTIVE)
>>>> + return IOMMU_DOMAIN_DMA;
>>>
>>> Is this specific to Intel? It seems like it could easily be done
>>> commonly like the check for untrusted external devices.
>>
>> It is currently Intel only but that will change. I will look into what
>> you suggest.
>>
>>>
>>>> +
>>>> if (dev_is_pci(dev)) {
>>>> struct pci_dev *pdev = to_pci_dev(dev);
>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>>> index 808ab70d..d49b7dd 100644
>>>> --- a/drivers/iommu/iommu.c
>>>> +++ b/drivers/iommu/iommu.c
>>>> @@ -23,6 +23,7 @@
>>>> #include <linux/property.h>
>>>> #include <linux/fsl/mc.h>
>>>> #include <linux/module.h>
>>>> +#include <linux/slaunch.h>
>>>> #include <trace/events/iommu.h>
>>>> static struct kset *iommu_group_kset;
>>>> @@ -2761,7 +2762,10 @@ void iommu_set_default_passthrough(bool cmd_line)
>>>> {
>>>> if (cmd_line)
>>>> iommu_cmd_line |= IOMMU_CMD_LINE_DMA_API;
>>>> - iommu_def_domain_type = IOMMU_DOMAIN_IDENTITY;
>>>> +
>>>> + /* Do not allow identity domain when Secure Launch is configured */
>>>> + if (!(slaunch_get_flags() & SL_FLAG_ACTIVE))
>>>> + iommu_def_domain_type = IOMMU_DOMAIN_IDENTITY;
>>>
>>> Quietly ignoring the setting and possibly leaving iommu_def_domain_type
>>> uninitialised (note that 0 is not actually a usable type) doesn't seem
>>> great. AFAICS this probably warrants similar treatment to the
>>
>> Ok so I guess it would be better to set it to IOMMU_DOMAIN_DMA event
>> though passthrough was requested. Or perhaps something more is needed here?
>>
>>> mem_encrypt_active() case - there doesn't seem a great deal of value in
>>> trying to save users from themselves if they care about measured boot
>>> yet explicitly pass options which may compromise measured boot. If you
>>> really want to go down that route there's at least the sysfs interface
>>> you'd need to nobble as well, not to mention the various ways of
>>> completely disabling IOMMUs...
>>
>> Doing a secure launch with the kernel is not a general purpose user use
>> case. A lot of work is done to secure the environment. Allowing
>> passthrough mode would leave the secure launch kernel exposed to DMA. I
>> think what we are trying to do here is what we intend though there may
>> be a better way or perhaps it is incomplete as you suggest.
>>
>
> I don't really like all these special cases. Generically, what you're
> trying to do is (AFAICT) to get the kernel to run in a mode in which
> it does its best not to trust attached devices. Nothing about this is
> specific to Secure Launch. There are plenty of scenarios in which
> this the case:
>
> - Virtual devices in a VM host outside the TCB, e.g. VDUSE, Xen
> device domains (did I get the name right), whatever tricks QEMU has,
> etc.
> - SRTM / DRTM technologies (including but not limited to Secure
> Launch -- plain old Secure Boot can work like this too).
> - Secure guest technologies, including but not limited to TDX and SEV.
> - Any computer with a USB-C port or other external DMA-capable port.
> - Regular computers in which the admin wants to enable this mode for
> whatever reason.
>
> Can you folks all please agree on a coordinated way for a Linux kernel
> to configure itself appropriately? Or to be configured via initramfs,
> boot option, or some other trusted source of configuration supplied at
> boot time? We don't need a whole bunch of if (TDX), if (SEV), if
> (secure launch), if (I have a USB-C port with PCIe exposed), if
> (running on Xen), and similar checks all over the place.
>

I replied to Robin Murphy in another thread. As far as the IOMMU is
concerned, I think we need to rethink our approach. As to the other
technologies you mention here, we have not considered special casing
anything at this point.

Thanks
Ross