Re: [PATCH] wan: fix a missing check of spi_write_then_read

[David Miller <davem@davemloft.net>]

From: Kangjie Lu <kjlu@umn.edu>
Date: Tue, 25 Dec 2018 22:34:41 -0600

> When spi_write_then_read() fails, "data" can be uninitialized and thus
> may contain a random value; the following execution checks "data" with a
> mask, the result could be random.
> 
> The fix inserts a check of spi_write_then_read(): if it fails, always
> returns -1.
> 
> Signed-off-by: Kangjie Lu <kjlu@umn.edu>
> ---
>  drivers/net/wan/slic_ds26522.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wan/slic_ds26522.c b/drivers/net/wan/slic_ds26522.c
> index 1f6bc8791d51..89681910d875 100644
> --- a/drivers/net/wan/slic_ds26522.c
> +++ b/drivers/net/wan/slic_ds26522.c
> @@ -64,8 +64,9 @@ static u8 slic_read(struct spi_device *spi, u16 addr)
>  	temp[0] = (u8)(((addr >> 8) & 0x7f) | 0x80);
>  	temp[1] = (u8)(addr & 0xfe);
>  
> -	spi_write_then_read(spi, &temp[0], SLIC_TWO_LEN, &data,
> -			    SLIC_TRANS_LEN);
> +	if (spi_write_then_read(spi, &temp[0], SLIC_TWO_LEN, &data,
> +			    SLIC_TRANS_LEN))
> +		return -1;

This makes no sense, all callers use the data value as-is and perform
no error checking.

You have to either accept this code to stay as it is, or adjust all of
the call sites to do error handling.

And that's not easy, -1 is a valud return value.