linux-hardening - Re: [RFC PATCH] um: replace deprecated strncpy with strscpy

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Message-ID: <202506092114.01147457A5@keescook>
Date: Mon, 9 Jun 2025 21:15:49 -0700
From: Kees Cook <kees@...nel.org>
To: Brahmajit Das <listout@...tout.xyz>
Cc: linux-hardening@...r.kernel.org, justinstitt@...gle.com, richard@....at,
	anton.ivanov@...bridgegreys.com, johannes@...solutions.net,
	linux-um@...ts.infradead.org
Subject: Re: [RFC PATCH] um: replace deprecated strncpy with strscpy

On Sun, Jun 08, 2025 at 03:00:06AM +0530, Brahmajit Das 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.
> 
> This modification is mainly due to the concerns on
> https://github.com/KSPP/linux/issues/336, where it was mentioned that
> strncpy_from_user is confusingly named as it does not NUL-pad the
> destination, but it does NOT guarantee NUL-termination.
> With this approach/patch we can always ensure that the dst buffer is NUL
> terminated.

I think we need wholesale fix the kernel's usage of
strncpy_from_user()... first, how is it being used? Are things being
manually terminated? Are something destinations not actually C Strings?
We may want two APIs (like strtomem vs strscpy). And then since we're
dealing with user data, I would think padding should be included?

-- 
Kees Cook