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Date:   Sun, 15 Oct 2023 17:06:19 +0200
From:   Simon Horman <horms@kernel.org>
To:     Justin Stitt <justinstitt@google.com>
Cc:     Thomas Sailer <t.sailer@alumni.ethz.ch>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-hams@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] hamradio: replace deprecated strncpy with strscpy
Message-ID: <20231015150619.GC1386676@kernel.org>
References: <20231012-strncpy-drivers-net-hamradio-baycom_epp-c-v1-1-8f4097538ee4@google.com>
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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@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>

...