Re: [PATCH 2/2] of: fdt: fix oob writes with large ftd properties

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 31.01.24 17:57, Stefan Kerkmann wrote:
> OOB writes can be triggered when fdt->dt_size * 2 is still smaller than
> the property for which memory should be allocated. This can happen under
> rare circumstances when editing a fdt with the of_property command and a
> property is larger than 128k in size.
> 
> This happend when editing a FIT image (which is a ftd) with the
> of_property command and the Kernel image was around 8M in size.
> 
> The simplified call chain is the following:
> 
> of_property -> of_flatten_dtb -> create new fdt with 64k in size (this is
> fixed) -> __of_flatten_dtb -> attempt to copy kernel image (8M) ->
> fdt_ensure_space -> allocate only 128k for fdt->dt -> memcopy 8M into fdt->dt
> buffer -> crash
> 
> The fix is to grow fdt->dt to hold at least the new property. The power
> of 2 increment is untouched to keep the same behaviour otherwise.
> 
> Signed-off-by: Stefan Kerkmann <s.kerkmann@pengutronix.de>
> ---
>  drivers/of/fdt.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index 4f79a6120f..1f24ed0bbc 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -399,6 +399,10 @@ static int fdt_ensure_space(struct fdt *fdt, int dtsize)
>  		previous = fdt->dt;
>  		new_size = fdt->dt_size * 2;
>  
> +		while (new_size <= dtsize) {
> +			new_size *= 2;
> +		}

A nitpick that I solely note because I already had feedback on the first patch:
Kernel coding style is to omit { braces } for single statement blocks.

In your case you could just do:

 if (new_size <= dtsize)
 	new_size = roundup_pow_of_two(new_size + dtsize);

I think to skip the loop.

Cheers,
Ahmad

> +
>  		if ((fdt->dt = memalign_realloc(previous, fdt->dt_size,
>  						new_size)) == NULL) {
>  			free(previous);
> 

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