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

[Stefan Kerkmann <s.kerkmann@pengutronix.de>]

Hello Ahmad,

On 31.01.24 18:21, Ahmad Fatoum wrote:
> 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.
> 

Thanks! That is the better solution.

To not over provision memory I changed the new size to be 
`roundup_pow_of_two(fdt->dt_size + dtsize)` as we know for sure that 
`dtsize` is already larger than `fdt->dt_size * 2`.

In (made up) case that we already have 8k space for the fdt and got a 
17k property we would allocate 65k (8k + 8k + 17k = 33k ⇾ rounded ⇾ 65k) 
and only 32k (8k + 17k = 25k → rounded → 32k) with `fdt->dt_size + dtsize`.

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

Cheers
Stefan

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