Re: [PATCH 8/8] spi: dw: change snprintf to scnprintf for possible overflow

[Kees Cook]

On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@xxxxxx> wrote:
>
> From: Silvio Cesare <silvio.cesare@xxxxxxxxx>
>
> Change snprintf to scnprintf. There are generally two cases where using
> snprintf causes problems.
>
> 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
> In this case, if snprintf would have written more characters than what the
> buffer size (SIZE) is, then size will end up larger than SIZE. In later
> uses of snprintf, SIZE - size will result in a negative number, leading
> to problems. Note that size might already be too large by using
> size = snprintf before the code reaches a case of size += snprintf.
>
> 2) If size is ultimately used as a length parameter for a copy back to user
> space, then it will potentially allow for a buffer overflow and information
> disclosure when size is greater than SIZE. When the size is used to index
> the buffer directly, we can have memory corruption. This also means when
> size = snprintf... is used, it may also cause problems since size may become
> large.  Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
> configuration.
>
> The solution to these issues is to use scnprintf which returns the number of
> characters actually written to the buffer, so the size variable will never
> exceed SIZE.
>
> Signed-off-by: Silvio Cesare <silvio.cesare@xxxxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Greg KH <greg@xxxxxxxxx>
> Signed-off-by: Willy Tarreau <w@xxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

>
> ---
>  drivers/spi/spi-dw.c | 36 ++++++++++++++++++------------------
>  1 file changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
> index b705f2bdb8b9..008d52d37439 100644
> --- a/drivers/spi/spi-dw.c
> +++ b/drivers/spi/spi-dw.c
> @@ -54,41 +54,41 @@ static ssize_t dw_spi_show_regs(struct file *file, char __user *user_buf,
>         if (!buf)
>                 return 0;
>
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "%s registers:\n", dev_name(&dws->master->dev));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "=================================\n");
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "CTRL0: \t\t0x%08x\n", dw_readl(dws, DW_SPI_CTRL0));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "CTRL1: \t\t0x%08x\n", dw_readl(dws, DW_SPI_CTRL1));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "SSIENR: \t0x%08x\n", dw_readl(dws, DW_SPI_SSIENR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "SER: \t\t0x%08x\n", dw_readl(dws, DW_SPI_SER));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "BAUDR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_BAUDR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "TXFTLR: \t0x%08x\n", dw_readl(dws, DW_SPI_TXFLTR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "RXFTLR: \t0x%08x\n", dw_readl(dws, DW_SPI_RXFLTR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "TXFLR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_TXFLR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "RXFLR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_RXFLR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "SR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_SR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "IMR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_IMR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "ISR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_ISR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "DMACR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_DMACR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "DMATDLR: \t0x%08x\n", dw_readl(dws, DW_SPI_DMATDLR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "DMARDLR: \t0x%08x\n", dw_readl(dws, DW_SPI_DMARDLR));
> -       len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> +       len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
>                         "=================================\n");
>
>         ret = simple_read_from_buffer(user_buf, count, ppos, buf, len);
> --
> 2.19.2
>


-- 
Kees Cook