[PATCH for-4.22 v4] dom0less: Prevent division by zero in handle_passthrough_prop()

[Dmytro Prokopchuk1 <dmytro_prokopchuk1@xxxxxxxx>]

A malformed 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 inside
handle_passthrough_prop():

    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 both 'address_cells' and 'size_cells'
are within the valid range of [1, 2] at the read side in scan_pfdt_node()
immediately after they are parsed. Any invalid cell size combination is
safely rejected early with an error message and return -EINVAL.

Fixes: 9ce974c47588 ("xen/arm: assign devices to boot domains")
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
---
Changes in v4:
- put the expressions in brackets
- improve the message as Andrew suggested
---
 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..47465a3609 100644
--- a/xen/common/device-tree/dom0less-build.c
+++ b/xen/common/device-tree/dom0less-build.c
@@ -341,6 +341,13 @@ static int __init scan_pfdt_node(struct kernel_info 
*kinfo, const void *pfdt,
     size_cells = device_tree_get_u32(pfdt, nodeoff, "#size-cells",
                                      DT_ROOT_NODE_SIZE_CELLS_DEFAULT);
 
+    if ( (address_cells < 1) || (address_cells > 2) ||
+         (size_cells < 1) || (size_cells > 2) )
+    {
+        dprintk(XENLOG_ERR "Invalid address_cells %u or size_cells %u\n");
+        return -EINVAL;
+    }
+
     node_next = fdt_first_subnode(pfdt, nodeoff);
     while ( node_next > 0 )
     {
-- 
2.43.0