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

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Message-ID: <202310261022.1B5587464E@keescook>
Date:   Thu, 26 Oct 2023 10:23:42 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Johnson <quic_jjohnson@...cinc.com>
Cc:     Justin Stitt <justinstitt@...gle.com>,
        Kalle Valo <kvalo@...nel.org>, 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 Tue, Oct 17, 2023 at 03:51:58PM -0700, Jeff Johnson wrote:
> 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

Yeah, that seems like a good refactor to do at the same time.

> > -	strncpy(extra, local->config.nodeName, 16);
> > -	extra[16] = '\0';
> > +	strscpy_pad(extra, local->config.nodeName, 16);

Justin, can you respin this with the open-coded "16" updated?

-Kees

-- 
Kees Cook