[PATCH v5 14/14] vpci: add TODO for the registers not explicitly handled

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

For unprivileged guests vpci_{read|write} need to be re-worked
to not passthrough accesses to the registers not explicitly handled
by the corresponding vPCI handlers: without fixing that passthrough
to guests is completely unsafe as Xen allows them full access to
the registers.

Xen needs to be sure that every register a guest accesses is not
going to cause the system to malfunction, so Xen needs to keep a
list of the registers it is safe for a guest to access.

For example, we should only expose the PCI capabilities that we know
are safe for a guest to use, i.e.: MSI and MSI-X initially.
The rest of the capabilities should be blocked from guest access,
unless we audit them and declare safe for a guest to access.

As a reference we might want to look at the approach currently used
by QEMU in order to do PCI passthrough. A very limited set of PCI
capabilities known to be safe for untrusted access are exposed to the
guest and registers need to be explicitly handled or else access is
rejected. Xen needs a fairly similar model in vPCI or else none of
this will be safe for unprivileged access.

Add the corresponding TODO comment to highlight there is a problem that
needs to be fixed.

Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

---
New in v5
---
 xen/drivers/vpci/vpci.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
index bdc8c63f73fa..4fb77d08825a 100644
--- a/xen/drivers/vpci/vpci.c
+++ b/xen/drivers/vpci/vpci.c
@@ -493,6 +493,29 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, 
unsigned int size)
     if ( !pdev->vpci )
     {
         spin_unlock(&pdev->vpci_lock);
+        /*
+         * TODO: for unprivileged guests vpci_{read|write} need to be re-worked
+         * to not passthrough accesses to the registers not explicitly handled
+         * by the corresponding vPCI handlers: without fixing that passthrough
+         * to guests is completely unsafe as Xen allows them full access to
+         * the registers.
+         *
+         * Xen needs to be sure that every register a guest accesses is not
+         * going to cause the system to malfunction, so Xen needs to keep a
+         * list of the registers it is safe for a guest to access.
+         *
+         * For example, we should only expose the PCI capabilities that we know
+         * are safe for a guest to use, i.e.: MSI and MSI-X initially.
+         * The rest of the capabilities should be blocked from guest access,
+         * unless we audit them and declare safe for a guest to access.
+         *
+         * As a reference we might want to look at the approach currently used
+         * by QEMU in order to do PCI passthrough. A very limited set of PCI
+         * capabilities known to be safe for untrusted access are exposed to 
the
+         * guest and registers need to be explicitly handled or else access is
+         * rejected. Xen needs a fairly similar model in vPCI or else none of
+         * this will be safe for unprivileged access.
+         */
         return vpci_read_hw(sbdf, reg, size);
     }
 
-- 
2.25.1