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Date: Thu, 19 Oct 2023 17:18:41 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] nvdimm/btt: replace deprecated strncpy with strscpy

On Thu, Oct 19, 2023 at 05:54:15PM +0000, Justin Stitt wrote:
> Found with grep.
> 
> 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 super->signature to be NUL-terminated based on its usage with
> memcmp against a NUL-term'd buffer:
> btt_devs.c:
> 253 | if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
> btt.h:
> 13  | #define BTT_SIG "BTT_ARENA_INFO\0"
> 
> NUL-padding is not required as `super` is already zero-allocated:
> btt.c:
> 985 | super = kzalloc(sizeof(struct btt_sb), GFP_NOIO);
> ... rendering any additional NUL-padding superfluous.
> 
> 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.
> 
> Let's also use the more idiomatic strscpy usage of (dest, src,
> sizeof(dest)) instead of (dest, src, XYZ_LEN) for buffers that the
> compiler can determine the size of. This more tightly correlates the
> destination buffer to the amount of bytes copied.
> 
> Side note, this pattern of memcmp() on two NUL-terminated strings should
> really be changed to just a strncmp(), if i'm not mistaken? I see
> multiple instances of this pattern in this system:
> 
> |       if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
> |               return false;
> 
> where BIT_SIG is defined (weirdly) as a double NUL-terminated string:
> 
> |       #define BTT_SIG "BTT_ARENA_INFO\0"
> 
> 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: Kees Cook <keescook@...omium.org>

-- 
Kees Cook

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