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

From: Srinivas Kandagatla
Date: Wed Oct 30 2024 - 07:43:53 EST




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