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Date: Sun, 15 Oct 2023 17:06:19 +0200
From: Simon Horman <horms@...nel.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Thomas Sailer <t.sailer@...mni.ethz.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	linux-hams@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] hamradio: replace deprecated strncpy with strscpy

On Thu, Oct 12, 2023 at 09:33:32PM +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.
> 
> We expect both hi.data.modename and hi.data.drivername to be
> NUL-terminated but not necessarily NUL-padded which is evident by its
> usage with sprintf:
> |       sprintf(hi.data.modename, "%sclk,%smodem,fclk=%d,bps=%d%s",
> |               bc->cfg.intclk ? "int" : "ext",
> |               bc->cfg.extmodem ? "ext" : "int", bc->cfg.fclk, bc->cfg.bps,
> |               bc->cfg.loopback ? ",loopback" : "");
> 
> Note that this data is copied out to userspace with:
> |       if (copy_to_user(data, &hi, sizeof(hi)))
> ... however, the data was also copied FROM the user here:
> |       if (copy_from_user(&hi, data, sizeof(hi)))

Thanks Justin,

I see that too.

Perhaps I am off the mark here, and perhaps it's out of scope for this
patch, but I do think it would be nicer if the kernel only sent
intended data to user-space, even if any unintended payload came
from user-space.

> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

...

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