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

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Date: Sat, 14 Oct 2023 18:13:18 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Florian Fainelli <f.fainelli@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] net: cpmac: replace deprecated strncpy with strscpy

On Thu, Oct 12, 2023 at 08:53:30PM +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 mdio_bus_id to be NUL-terminated based on its use with
> snprint():
> |       snprintf(priv->phy_name, MII_BUS_ID_SIZE, PHY_ID_FMT,
> |                                               mdio_bus_id, phy_id);
> 
> Moreover, as this is the only use of mdio_bus_id, we can see that
> NUL-padding is not required.
> 
> 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.
> 
> Note that for the replacement involving cpmac_mii->id, the source
> buffer's length is equal to MII_BUS_ID_SIZE which could result in a
> buffer overread. However, there is no buffer overread since "cpmac-1"
> (the string copied into cpmac_mii->id) is smaller than MII_BUS_ID_SIZE
> thus meaning the previous usage of strncpy() here did _not_ have any
> overread bugs. Nonetheless, let's still favor strscpy() over strncpy().
> 
> 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>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew