[PATCH for-4.22 v2] dom0less: Prevent division by zero in handle_passthrough_prop()

[Dmytro Prokopchuk1 <dmytro_prokopchuk1@xxxxxxxx>]

A malformed provided partial DTB specifying both '#address-cells = <0>'
and '#size-cells = <0>' causes '(address_cells * 2 + size_cells)' to
evaluate to 0. This sum is subsequently used as a divisor when calculating
the number of regions in the 'xen,reg' property:

    len = fdt32_to_cpu(xen_reg->len) / ((address_cells * 2 + size_cells) *
                                        sizeof(uint32_t));

This leads to a division by zero exception in the Xen hypervisor during
boot, causing a hypervisor panic/crash.

Fix this by validating that '(address_cells * 2 + size_cells)' is greater
than zero before performing the division. If it is zero, log an error
message and return -EINVAL.

Fixes: 9ce974c47588 ("xen/arm: assign devices to boot domains")
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in v2:
- added Fix tag
- added Oleksii's R-b tags
---
 xen/common/device-tree/dom0less-build.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/common/device-tree/dom0less-build.c 
b/xen/common/device-tree/dom0less-build.c
index eacfd93087..6796851844 100644
--- a/xen/common/device-tree/dom0less-build.c
+++ b/xen/common/device-tree/dom0less-build.c
@@ -154,6 +154,13 @@ static int __init handle_passthrough_prop(struct 
kernel_info *kinfo,
 
     /* xen,reg specifies where to map the MMIO region */
     cell = (const __be32 *)xen_reg->data;
+
+    if ( (address_cells * 2 + size_cells) == 0 )
+    {
+        printk(XENLOG_ERR "Invalid address/size cells combination (both 0)\n");
+        return -EINVAL;
+    }
+
     len = fdt32_to_cpu(xen_reg->len) / ((address_cells * 2 + size_cells) *
                                         sizeof(uint32_t));
 
-- 
2.43.0