Re: [PATCH v2 1/3] nvmem: core: improve range check for nvmem_cell_write()

[Srinivas Kandagatla <srinivas.kandagatla@linaro.org>]

On 29/10/2024 21:31, Jennifer Berringer wrote:
> On 10/29/24 13:55, Srinivas Kandagatla wrote:
>> if (!nvmem || nvmem->read_only || len != cell->bytes)
>>      return -EINVAL;
>>
>> Does this work?
>>
>> --srini
> 
> I decided against this because it seems potentially useful to allow len to be less than cell->bytes when bit_offset is nonzero. I assumed that was the purpose of the original "cell->bit_offset == 0".
> 
I don't think we support this case.

The reason why this check was initially added is,

If we have bit_offset as non zero or nbits set, cell->bytes is can be 
different to the actual space that is available in the cell, Ex: 2 bits 
with offset of 7 might end up taking 2 bytes. So the existing check is 
correct as it is and valid for cases where the bit_offset is 0.

In this particular case the right solution to the issue is to add more 
sanity checks in case bit_offset is non zero.


This change should help, can you pl  try it.

---------------------------->cut<-----------------------------
diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 90c46f6e465d..e6d91a9a9dc5 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1780,6 +1780,9 @@ static int __nvmem_cell_entry_write(struct 
nvmem_cell_entry *cell, void *buf, si
                 return -EINVAL;

         if (cell->bit_offset || cell->nbits) {
+               if (BITS_TO_BYTES(cell->nbits) != len)
+                       return -EINVAL;
+
                 buf = nvmem_cell_prepare_write_buffer(cell, buf, len);
                 if (IS_ERR(buf))
                         return PTR_ERR(buf);
---------------------------->cut<-----------------------------

thanks,
srini



> For example, if a cell entry has the following field values
>      { .bit_offset = 4, .nbits = 8, .bytes = 2, ...}
> then it would make sense to call nvmem_cell_write() with len=1 in order to write 8 bits. To allow that, I used "len > cell->bytes" instead of "!=" later in this function:
> 
>>> @@ -1780,9 +1779,13 @@ static int __nvmem_cell_entry_write(struct nvmem_cell_entry *cell, void *buf, si
>>>            return -EINVAL;
>>>          if (cell->bit_offset || cell->nbits) {
>>> +        if (len > cell->bytes)
>>> +            return -EINVAL;
>>>            buf = nvmem_cell_prepare_write_buffer(cell, buf, len);
>>>            if (IS_ERR(buf))
>>>                return PTR_ERR(buf);
>>> +    } else if (len != cell->bytes) {
>>> +        return -EINVAL;
>>>        }
> 
> If you disagree with my reasoning then yes, your suggestion works and I can use that instead of what I wrote. None of the current in-tree callers of this function rely on that possibility I described.
> 
> Thank you for the feedback.
> 
> -Jennifer
>