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

[Srinivas Kandagatla]

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