linux-kernel - Re: [PATCH v3] arm64/sysreg: refactor deprecated strncpy

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Message-ID: <ZPHUb4svC//EhyqJ@google.com>
Date:   Fri, 1 Sep 2023 12:09:19 +0000
From:   Mostafa Saleh <smostafa@...gle.com>
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,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] arm64/sysreg: refactor deprecated strncpy

Hi Justin,

On Thu, Aug 31, 2023 at 10:55:59PM +0000, Justin Stitt wrote:
> strncpy is deprecated [1] and should not be used if the src string is
> not NUL-terminated.
> 
> When dealing with `cmdline` we are counting the number of characters
> until a space then copying these over into `buf`. Let's not use any of
> the str*() functions since the src string is not necessarily NUL-terminated.
> 
> Prefer `memcpy()` alongside a forced NUL-termination as it more
> accurately describes what is going on within this function, i.e: copying
> from non NUL-terminated buffer into a NUL-terminated buffer.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Suggested-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Here's a quick rundown on the history of this patch:
> 1) v1 (changes requested)
> 2) v2 (applied to arm64 (for-next/misc))
> 3) v2 reverted (https://lore.kernel.org/all/20230831162227.2307863-1-smostafa@google.com/)

This is just a patch, it is not reverted yet, also Marc replied with
another proposal to use strscpy instead but with the correct length.

> 4) v3 (fixes problems with both v1 and v2)
> 
> Changes in v3:
> - Fix faulty logic and use memcpy over strscpy (thanks Mostafa and Kees)
> - Use '\0' instead of 0 to make it abundantly clear that `buf` is a NUL-terminated string
> - Link to v2: https://lore.kernel.org/r/20230811-strncpy-arch-arm64-v2-1-ba84eabffadb@google.com
> 
> Changes in v2:
> - Utilize return value from strscpy and check for truncation (thanks Kees)
> - Link to v1: https://lore.kernel.org/r/20230810-strncpy-arch-arm64-v1-1-f67f3685cd64@google.com
> ---
>  arch/arm64/kernel/idreg-override.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> index 2fe2491b692c..3addc09f8746 100644
> --- a/arch/arm64/kernel/idreg-override.c
> +++ b/arch/arm64/kernel/idreg-override.c
> @@ -263,8 +263,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
>  			return;
>  
>  		len = min(len, ARRAY_SIZE(buf) - 1);
> -		strncpy(buf, cmdline, len);
> -		buf[len] = 0;
> +		memcpy(buf, cmdline, len);
> +		buf[len] = '\0';
>  
>  		if (strcmp(buf, "--") == 0)
>  			return;
> 
> ---
> base-commit: 706a741595047797872e669b3101429ab8d378ef
> change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8

Your patch will not apply on mainline but on the revert, otherwise

Tested-by: Mostafa Saleh <smostafa@...gle.com>

> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>

Thanks,
Mostafa