[PATCH] mtd: nand: pxa3xx: Fix buffer overflow during raw reads

[Chris Packham]

Hi Pierre,

On Sun, Jul 30, 2023 at 6:08 AM Pierre Bourdon <delroth at gmail.com> wrote:
>
> Chunked raw reads get accumulated to the data buffer, but in some
> ECC configurations they can end up being larger than the originally
> computed size (write page size + OOB size). For example:
>
> 4K page size, ECC strength 8:
> - Normal reads: writesize (4096B) + oobsize (128B) = 4224 bytes.
> - Chunked raw reads: 4 chunks of 1024B + 1 final spare area of 64B + 5
>   ECC areas of 32B = 4320B.

I'm not a NAND expert and I haven't sat down and fully grasped the
math but I was curious to see what the Linux kernel did. It looks like
it uses the same mtd->writesize + mtd->oobsize calculation (see
nand_scan_tail() in nand_base.c). So either Linux has the same bug or
maybe there's something off in u-boot's nfc_layouts[]. I'll see if I
can get one of my boards to trigger a KASAN report (I'm not sure if
any of the NAND chips we use will hit the cases you're pointing out).

>
> Fixes: 6293b0361d9 ("mtd: nand: pxa3xx: add raw read support")
> Signed-off-by: Pierre Bourdon <delroth at gmail.com>
> ---
>
>  drivers/mtd/nand/raw/pxa3xx_nand.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mtd/nand/raw/pxa3xx_nand.c b/drivers/mtd/nand/raw/pxa3xx_nand.c
> index d502e967f9..2894ababbe 100644
> --- a/drivers/mtd/nand/raw/pxa3xx_nand.c
> +++ b/drivers/mtd/nand/raw/pxa3xx_nand.c
> @@ -1471,6 +1471,19 @@ static void pxa3xx_nand_detect_config(struct pxa3xx_nand_info *info)
>
>  static int pxa3xx_nand_init_buff(struct pxa3xx_nand_info *info)
>  {
> +       unsigned int chunk_size;
> +       unsigned int last_chunk_size;
> +
> +       /*
> +        * The data buffer needs to not only be large enough for normal + OOB
> +        * reads, but also for raw reads. The raw reads can end up taking more
> +        * space due to the chunking scheme.
> +        */
> +       chunk_size = info->chunk_size + info->spare_size + info->ecc_size;
> +       last_chunk_size =
> +               info->last_chunk_size + info->last_spare_size + info->ecc_size;
> +       info->buf_size = info->nfullchunks * chunk_size + last_chunk_size;
> +
>         info->data_buff = kmalloc(info->buf_size, GFP_KERNEL);
>         if (info->data_buff == NULL)
>                 return -ENOMEM;
> @@ -1661,7 +1674,6 @@ static int pxa3xx_nand_scan(struct mtd_info *mtd)
>         kfree(info->data_buff);
>
>         /* allocate the real data + oob buffer */
> -       info->buf_size = mtd->writesize + mtd->oobsize;
>         ret = pxa3xx_nand_init_buff(info);
>         if (ret)
>                 return ret;
> --
> 2.41.0
>