[PATCH v6] 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 range of [1, 2] at the top of handle_passthrough_prop().
Any invalid cell size combination is safely rejected early with an error
message and return -EINVAL.

Furthermore, update handle_passthrough_prop() to use the sizeof(*cell)
instead of sizeof(uint32_t).

Fixes: 9ce974c47588 ("xen/arm: assign devices to boot domains")
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
---
Changes in v6:
- move cells check at the top of handle_passthrough_prop() with a comment
- reword commit message
- replace sizeof(uint32_t) with sizeof(*cell) (use expression instead of type)
Test CI pipeline:
https://gitlab.com/xen-project/people/dimaprkp4k/xen/-/pipelines/2667596761
---
 xen/common/device-tree/dom0less-build.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/xen/common/device-tree/dom0less-build.c 
b/xen/common/device-tree/dom0less-build.c
index eacfd93087..9513c1c837 100644
--- a/xen/common/device-tree/dom0less-build.c
+++ b/xen/common/device-tree/dom0less-build.c
@@ -152,10 +152,23 @@ static int __init handle_passthrough_prop(struct 
kernel_info *kinfo,
             return -ENOMEM;
     }
 
+    /*
+     * xen,reg holds flat host/guest physical addresses and sizes, so the
+     * inherited #address-cells/#size-cells must each be 1 or 2. This also
+     * guards the len division below against a zero or wrapped divisor.
+     */
+    if ( (address_cells < 1) || (address_cells > 2) ||
+         (size_cells < 1) || (size_cells > 2) )
+    {
+        printk(XENLOG_ERR "Invalid address_cells %u or size_cells %u\n",
+               address_cells, size_cells);
+        return -EINVAL;
+    }
+
     /* xen,reg specifies where to map the MMIO region */
     cell = (const __be32 *)xen_reg->data;
     len = fdt32_to_cpu(xen_reg->len) / ((address_cells * 2 + size_cells) *
-                                        sizeof(uint32_t));
+                                        sizeof(*cell));
 
     for ( i = 0; i < len; i++ )
     {
-- 
2.43.0