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

[Ahmad Fatoum]

On 01.02.24 11:24, Stefan Kerkmann wrote:
> 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 at 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`.

Sounds good.

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

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