linux-kernel - Re: [PATCH v2] wifi: ipw2x00: replace deprecated strncpy with strscpy

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Date:   Wed, 18 Oct 2023 18:19:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Stanislav Yakovlev <stas.yakovlev@...il.com>,
        Kalle Valo <kvalo@...nel.org>, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] wifi: ipw2x00: replace deprecated strncpy with
 strscpy_pad

On Tue, Oct 17, 2023 at 09:48:15PM +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 string
> interfaces.
> 
> `extra` is intended to be NUL-terminated which is evident by the manual
> assignment of a NUL-byte as well as its immediate usage with strlen().
> 
> Moreover, many of these getters and setters are NUL-padding buffers with
> memset():
> 2439  |	memset(&tx_power, 0, sizeof(tx_power));
> 9998  | memset(sys_config, 0, sizeof(struct ipw_sys_config));
> 10084 | memset(tfd, 0, sizeof(*tfd));
> 10261 | memset(&dummystats, 0, sizeof(dummystats));
> ... let's maintain this behavior and NUL-pad our destination buffer.
> 
> 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.
> 
> To be clear, there is no bug in the current implementation as
> MAX_WX_STRING is much larger than the size of the string literals being
> copied from. Also, strncpy() does NUL-pad the destination buffer and
> using strscpy_pad() simply matches that behavior. All in all, there
> should be no functional change but we are one step closer to eliminating
> usage of strncpy().
> 
> Do note that we cannot use the more idiomatic strscpy invocation of
> (dest, src, sizeof(dest)) as the destination buffer cannot have its size
> determined at compile time. So, let's stick with (dest, src, LEN).

Yeah, these interfaces have external buffer size declarations. In this
case, MAX_WX_STRING.

This is probably one of the most difficult set of callbacks to track
down. sysfs might be worse...

But, ultimately, this is a private ioctl handler, and it is all boiled
down to calling ioctl_private_iw_point() (via ioctl_private_call()),
which does the allocation of "extra". The size, which is not passed to
the handler (*sob*), is determined by: get_priv_descr_and_size(), which
is looking at drivers/net/wireless/intel/ipw2x00/ipw2200.c's
ipw_priv_args:

        {
         .cmd = IPW_PRIV_GET_MODE,
         .get_args = IW_PRIV_TYPE_CHAR | IW_PRIV_SIZE_FIXED | MAX_WX_STRING,
         .name = "get_mode"},

So it looks good to me.

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook