linux-hardening - Re: [PATCH] arm64/sysreg: refactor deprecated strncpy

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Date:   Thu, 10 Aug 2023 12:00:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] arm64/sysreg: refactor deprecated strncpy

On Thu, Aug 10, 2023 at 06:39:03PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1]. Which seems to be the case here due to the forceful setting of `buf`'s
> tail to 0.

Another note to include in these evaluations would be "does the
destination expect to be %NUL padded?". Here, it looks like no, as all
the routines "buf" is passed to expect a regular C string (padding
doesn't matter).

> 
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on its destination buffer argument which is
> _not_ the case for `strncpy`!
> 
> In this case, there is some behavior being used in conjunction with
> `strncpy` that `strscpy` already implements. This means we can drop some
> of the extra stuff like `... -1` and `buf[len] = 0`
> 
> This should have no functional change and yet uses a more robust and
> less ambiguous interface whilst reducing code complexity.
> 
> Link: 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>
> ---
> For reference, see a part of `strscpy`'s implementation here:
> 
> |	/* Hit buffer length without finding a NUL; force NUL-termination. */
> |	if (res)
> |		dest[res-1] = '\0';
> 
> Note: compile tested
> ---
>  arch/arm64/kernel/idreg-override.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> index 2fe2491b692c..482dc5c71e90 100644
> --- a/arch/arm64/kernel/idreg-override.c
> +++ b/arch/arm64/kernel/idreg-override.c
> @@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
>  		if (!len)
>  			return;
>  
> -		len = min(len, ARRAY_SIZE(buf) - 1);
> -		strncpy(buf, cmdline, len);
> -		buf[len] = 0;
> +		len = min(len, ARRAY_SIZE(buf));
> +		strscpy(buf, cmdline, len);

This, however, isn't correct: "cmdline" will be incremented by "leN"
later, and we want a count of the characters copied into "buf", even if
they're truncated. I think this should be:

		strscpy(buf, cmdline, ARRAY_SIZE(buf));
		len = strlen(buf);

-Kees

>  
>  		if (strcmp(buf, "--") == 0)
>  			return;
> 
> ---
> base-commit: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
> change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook