linux-hardening - Re: [PATCH] airo: replace deprecated strncpy with strscpy

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Date: Tue, 17 Oct 2023 15:51:58 -0700
From: Jeff Johnson <quic_jjohnson@...cinc.com>
To: Justin Stitt <justinstitt@...gle.com>, Kalle Valo <kvalo@...nel.org>
CC: <linux-wireless@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] airo: replace deprecated strncpy with strscpy_pad

On 10/17/2023 2:12 PM, 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 string
> interfaces.
> 
> `extra` is clearly supposed to be NUL-terminated which is evident by the
> manual NUL-byte assignment as well as its immediate usage with strlen().
> 
> Moreover, let's NUL-pad since there is deliberate effort (48 instances)
> made elsewhere to zero-out buffers in these getters and setters:
> 6050 | memset(local->config.nodeName, 0, sizeof(local->config.nodeName));
> 6130 | memset(local->config.rates, 0, 8);
> 6139 | memset(local->config.rates, 0, 8);
> 6414 | memset(key.key, 0, MAX_KEY_SIZE);
> 6497 | memset(extra, 0, 16);
> (to be clear, strncpy also NUL-padded -- we are matching that behavior)
> 
> Considering the above, a suitable replacement is `strscpy_pad` due to
> the fact that it guarantees both NUL-termination and NUL-padding on the
> destination buffer.
> 
> Technically, we can now copy one less byte into `extra` as we cannot
> determine the sizeof `extra` at compile time and the hard-coded value of
> 16 means that strscpy_pad() will automatically truncate and set the byte
> at offset 15 to NUL. However, the current code manually sets a
> NUL-byte at offset 16. If this is an issue, the solution is to change
> the hard-coded magic number to 17 instead of 16. I didn't do this in
> this patch because a hard-coded 17 seems bad (even more so than 16).

this function is a wext handler. In wext-core.c we have:
static const struct iw_ioctl_description standard_ioctl[] = {
...
	[IW_IOCTL_IDX(SIOCGIWNICKN)] = {
		.header_type	= IW_HEADER_TYPE_POINT,
		.token_size	= 1,
		.max_tokens	= IW_ESSID_MAX_SIZE,
	},

So the buffer size is (strangely) IW_ESSID_MAX_SIZE if you want to use 
that for the buffer size

> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>   drivers/net/wireless/cisco/airo.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wireless/cisco/airo.c b/drivers/net/wireless/cisco/airo.c
> index dbd13f7aa3e6..8cfb1de5933e 100644
> --- a/drivers/net/wireless/cisco/airo.c
> +++ b/drivers/net/wireless/cisco/airo.c
> @@ -6067,8 +6067,7 @@ static int airo_get_nick(struct net_device *dev,
>   	struct airo_info *local = dev->ml_priv;
>   
>   	readConfigRid(local, 1);
> -	strncpy(extra, local->config.nodeName, 16);
> -	extra[16] = '\0';
> +	strscpy_pad(extra, local->config.nodeName, 16);
>   	dwrq->length = strlen(extra);
>   
>   	return 0;
> 
> ---
> base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
> change-id: 20231017-strncpy-drivers-net-wireless-cisco-airo-c-d09cd0500a6e
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>