linux-hardening - Re: [PATCH] ACPI: OSI: refactor deprecated strncpy

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Date:   Thu, 14 Sep 2023 20:16:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] ACPI: OSI: refactor deprecated strncpy

On Mon, Sep 11, 2023 at 08:36:44PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We know `osi->string` is a NUL-terminated string due to its eventual use
> in `acpi_install_interface()` and `acpi_remove_interface()` which expect
> a `acpi_string` which has been specifically typedef'd as:
> |  typedef char *acpi_string;	/* Null terminated ASCII string */
> 
> ... and which also has other string functions used on it like `strlen`.
> Furthermore, padding is not needed in this instance either.

Following the callers, I agree, this doesn't need %NUL padding -- it's
always processed as a regular C string.

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> 
> Due to the reasoning above a suitable replacement is `strscpy` [2] since
> it guarantees NUL-termination on the destination buffer and doesn't
> unnecessarily NUL-pad.
> 
> While there is unlikely to be a buffer overread (or other related bug)
> in this case, we should still favor a more robust and less ambiguous
> interface.
> 
> 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
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested
> ---
>  drivers/acpi/osi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/osi.c b/drivers/acpi/osi.c
> index d4405e1ca9b9..df9328c850bd 100644
> --- a/drivers/acpi/osi.c
> +++ b/drivers/acpi/osi.c
> @@ -110,7 +110,7 @@ void __init acpi_osi_setup(char *str)
>  			break;
>  		} else if (osi->string[0] == '\0') {
>  			osi->enable = enable;
> -			strncpy(osi->string, str, OSI_STRING_LENGTH_MAX);
> +			strscpy(osi->string, str, OSI_STRING_LENGTH_MAX);
>  			break;
>  		}
>  	}
> 
> ---
> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> change-id: 20230911-strncpy-drivers-acpi-osi-c-c801b7427987
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook