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

[Andy Lutomirski]

On Mon, Jun 21, 2021 at 10:51 AM Ross Philipson
<ross.philipson@xxxxxxxxxx> 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@xxxxxxxxxx>
> >> ---
> >>   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.