[PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Gustavo A. R. Silva]

On 2/12/21 08:12, Miquel Raynal wrote:
> Hi Gustavo,
> 
> "Gustavo A. R. Silva" <gustavoars at kernel.org> wrote on Fri, 12 Feb 2021
> 04:40:22 -0600:
> 
>> Cast &data to (char *) in order to avoid unintentionally accessing
>> the stack.
>>
>> Notice that data is of type u32, so any increment to &data
>> will be in the order of 4-byte chunks, and this piece of code
>> is actually intended to be a byte offset.
> 
> I don't have the same reading. I don't say that Coverity report is
> wrong, but let's discuss this a bit further.
> 
> Given that &data is of type u32 *, you say that "&data + shift"
> produces increments of 4-bytes, ie. we would access "&data + 4 *
> shift"? Because I don't think this is the case (again, I may be wrong).

Yep; this is pointer arithmetic. If you have an object ptr of type u32 *:

u32 *ptr;

and let's say it points to address 100. If you increment it by one:

ptr++

ptr will now point to address 104, not to 101.

Now, if instead, you first cast ptr to 'char *' and increment it by 1,
then it will point to address 101.

> 
> I'm sure this would be the case if we dereferenced data like
> data[shift] though, but in this case I don't see what this cast is
> fixing. Can you enlighten me?
> 
> Could the out-of-bounds warning come from the fact that shift
> might be bigger than the data array spread?

Whats Coverity wants to say here is basically that using &data as an
array might corrupt adjacent memory.

Thanks
--
Gustavo

> 
>> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
>> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
>> Cc: stable at vger.kernel.org
>> Signed-off-by: Gustavo A. R. Silva <gustavoars at kernel.org>
>> ---
>>  drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
>> index a35450002284..58782cfaf71c 100644
>> --- a/drivers/mtd/maps/physmap-bt1-rom.c
>> +++ b/drivers/mtd/maps/physmap-bt1-rom.c
>> @@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
>>  	if (shift) {
>>  		chunk = min_t(ssize_t, 4 - shift, len);
>>  		data = readl_relaxed(src - shift);
>> -		memcpy(to, &data + shift, chunk);
>> +		memcpy(to, (char *)&data + shift, chunk);
>>  		src += chunk;
>>  		to += chunk;
>>  		len -= chunk;
> 
> Thanks,
> Miquèl
>