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Re: [Xen-devel] [PATCH v4] gnttab: don't expose host physical address without need


On 05/12/2019 15:34, Jan Beulich wrote:
Translated domains shouldn't see host physical addresses. While the
address is also not supposed to be handed back even to non-translated
domains when GNTMAP_device_map is not set (as explicitly stated by a
comment in the public header), PV kernels (Linux at least) assume the
field to get populated nevertheless. (Similarly mapkind() should check
only GNTMAP_device_map.)

Along these lines split the paging mode related check near the top of
map_grant_ref() to handle the "external" and "translated" cases
separately (GNTMAP_device_map use getting tied to being non-translated
rather than non-external).

Still along these lines in the unmapping case there's no point checking
->dev_bus_addr when GNTMAP_device_map isn't set (and hence the field
isn't going to be consumed).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
v4: Re-base over dropped patches.
v3: New.

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -967,10 +967,16 @@ map_grant_ref(
if ( unlikely(paging_mode_external(ld) &&
-                  (op->flags & (GNTMAP_device_map|GNTMAP_application_map|
-                            GNTMAP_contains_pte))) )
+                  (op->flags & (GNTMAP_application_map|GNTMAP_contains_pte))) )
-        gdprintk(XENLOG_INFO, "No device mapping in HVM domain\n");
+        gdprintk(XENLOG_INFO, "No app/pte mapping in HVM domain\n");
+        op->status = GNTST_general_error;
+        return;
+    }
+    if ( paging_mode_translate(ld) && unlikely(op->flags & GNTMAP_device_map) )

There is at least one instance in Linux where GNTMAP_device_map may be given regardless the type of the guest. See dmabuf_exp_from_refs() in drivers/xen/gntdev-dmabuf.c.

How are you going to deal with them?

+    {
+        gdprintk(XENLOG_INFO, "No device mapping in translated domain\n");
          op->status = GNTST_general_error;
@@ -1213,7 +1219,8 @@ map_grant_ref(
      if ( need_iommu )
          double_gt_unlock(lgt, rgt);
- op->dev_bus_addr = mfn_to_maddr(mfn);
+    op->dev_bus_addr = paging_mode_translate(ld) ? op->host_addr
+                                                 : mfn_to_maddr(mfn);

The "host_addr" is pretty confusing. I first thought it would be a "Host Physical Address" but it seems to be a "Guest Physical address"

If so, this is going to break Linux Dom0 on Arm where it is expected to return the machine physical address to have a DMA fully working.

I don't abide with the current behavior on Arm, but I don't think we should break them when there are no replacement for it.

So it would be better if we look at a different approach (i.e a new flag or strict mode) in order to avoid breakage.


Julien Grall

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