Re: [PATCH master] of: fdt: fix overflows when parsing sizes

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

On 24.07.24 10:36, Sascha Hauer wrote:
> On Mon, Jul 22, 2024 at 07:24:10PM +0200, Ahmad Fatoum wrote:
>> The function dt_struct_advance() is used to advance a pointer to the next
>> offset within the structure block, while checking that the result is in
>> bounds.
>>
>> Unfortunately, the function used a signed size argument. This had the
>> effect that a too-large size in the FDT wrapped around and caused the
>> pointer to move backwards.
>>
>> This issue was found by libfuzzer which generated an FDT that
>> always triggered an out-of-memory condition: One struct indicated a size
>> that caused the pointer to move backwards.
>>
>> The resulting loop allocated memory on every iteration and eventually
>> ran out.
>>
>> Fix this by using unsigned sizes and treating wrap around as an
>> error case.
>>
>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>> ---
>>  drivers/of/fdt.c | 11 ++++++-----
>>  1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>> index 8dca41990c87..237468cd8164 100644
>> --- a/drivers/of/fdt.c
>> +++ b/drivers/of/fdt.c
>> @@ -32,12 +32,13 @@ static inline bool __dt_ptr_ok(const struct fdt_header *fdt, const void *p,
>>  }
>>  #define dt_ptr_ok(fdt, p) __dt_ptr_ok(fdt, p, sizeof(*(p)), __alignof__(*(p)))
>>  
>> -static inline uint32_t dt_struct_advance(struct fdt_header *f, uint32_t dt, int size)
>> +static inline uint32_t dt_struct_advance(struct fdt_header *f, uint32_t dt, uint32_t size)
>>  {
>> -	dt += size;
>> -	dt = ALIGN(dt, 4);
>> +	if (check_add_overflow(dt, size, &dt))
>> +		return 0;
>>  
>> -	if (dt > f->off_dt_struct + f->size_dt_struct)
>> +	dt = ALIGN(dt, 4);
>> +	if ((!dt && size) || dt > f->off_dt_struct + f->size_dt_struct)
>>  		return 0;
> 
> I am not sure I fully understand the newly added (!dt && size).
> 
> I think it's for the case when the initial addition results in something
> like 0xfffffffe and the ALIGN(dt, 4) makes dt become 0, right?

Exactly.

> I think dt being zero is a an error anyway, so what is the && size good
> for?

You're right. I will drop the check for v2.

> 
>>  
>>  	return dt;
> 
> When dt is zero it is returned here which will be considered an error by
> the caller anyway, so it seems the (!dt && size) check doesn't add
> anything.
> 
> Note we have dt_struct_advance() twice in the tree. Care to fix the
> other place as well?

Will do.

Thanks,
Ahmad

> 
> Sascha
> 

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