Subject: Re: [PATCH] net: dsa: lan9303: replace deprecated strncpy with memcpy

From: Kees Cook <[email-masked]>
Date: Wed, 4 Oct 2023 20:07:55 -0700

On Thu, Oct 05, 2023 at 12:30:18AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous
> interfaces.
> 
> Let's opt for memcpy as we are copying strings into slices of length
> `ETH_GSTRING_LEN` within the `data` buffer. Other similar get_strings()
> implementations [2] [3] use memcpy().
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://elixir.bootlin.com/linux/v6.3/source/drivers/infiniband/ulp/opa_vnic/opa_vnic_ethtool.c#L167 [2]
> Link: https://elixir.bootlin.com/linux/v6.3/source/drivers/infiniband/ulp/ipoib/ipoib_ethtool.c#L137 [3]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: [email-masked]
> Signed-off-by: Justin Stitt <[email-masked]>
> ---
> Note: build-tested only.
> ---
>  drivers/net/dsa/lan9303-core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/dsa/lan9303-core.c b/drivers/net/dsa/lan9303-core.c
> index ee67adeb2cdb..665d69384b62 100644
> --- a/drivers/net/dsa/lan9303-core.c
> +++ b/drivers/net/dsa/lan9303-core.c
> @@ -1013,8 +1013,8 @@ static void lan9303_get_strings(struct dsa_switch *ds, int port,
>  		return;
>  
>  	for (u = 0; u < ARRAY_SIZE(lan9303_mib); u++) {
> -		strncpy(data + u * ETH_GSTRING_LEN, lan9303_mib[u].name,
> -			ETH_GSTRING_LEN);
> +		memcpy(data + u * ETH_GSTRING_LEN, lan9303_mib[u].name,
> +		       ETH_GSTRING_LEN);

This won't work because lan9303_mib entries aren't ETH_GSTRING_LEN-long
strings; they're string pointers:

static const struct lan9303_mib_desc lan9303_mib[] = {
        { .offset = LAN9303_MAC_RX_BRDCST_CNT_0, .name = "RxBroad", },

So this really does need a strcpy-family function.

And, I think the vnic_gstrings_stats and ipoib_gstrings_stats examples
are actually buggy -- they're copying junk into userspace...

I am reminded of this patch, which correctly uses strscpy_pad():
https://lore.kernel.org/lkml/[email-masked]/

I think you want to do the same here, and use strscpy_pad(). And perhaps
send some fixes for the other memcpy() users?

(Have I mentioned I really dislike the get_strings() API?)

-Kees

-- 
Kees Cook