Re: [PATCH for-4.22 v5] dom0less: Prevent division by zero in handle_passthrough_prop()

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 09-Jul-26 14:31, Dmytro Prokopchuk1 wrote:
> 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 v5:
> - fixed mistakes around dprintk()
> Test CI pipeline:
> https://gitlab.com/xen-project/people/dimaprkp4k/xen/-/pipelines/2664600678
> ---
>  xen/common/device-tree/dom0less-build.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/xen/common/device-tree/dom0less-build.c 
> b/xen/common/device-tree/dom0less-build.c
> index eacfd93087..c054ea4e2f 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -341,6 +341,14 @@ 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) )
Too many discussions yesterday... Today, I looked at the complete flow once
again and I'm sorry to say that my previous comment to move the check right at
the read side was incorrect. #address/size-cells define the number of cells for
their children and these don't need to have xen,reg. Even our passthrough
example (docs/misc/arm/passthrough.txt) defines a ethernet node with #size-cells
= <0> for its PHY child with a 1 cell reg but no xen,reg. Here, for our sanity
check, we only care that number of cells for xen,reg is either 1 or 2. 0 is a
legitimate value for nodes without xen,reg (clock nodes, SPI, GPIO, etc.).
Values above 2 i.e. 3 is PCI only which does not go through this path but still,
we should only care about nodes with xen,reg.

Therefore, please move this check at the top of handle_passthrough_prop() with a
comment below (the number of discussions about this subject is a clear
indication that a comment is needed):

/*
 * 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.
 */

~Michal