linux-hardening - Re: [PATCH v2] um: refactor deprecated strncpy to strtomem

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Date:   Tue, 8 Aug 2023 17:42:48 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-hardening@...r.kernel.org, linux-um@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] um: refactor deprecated strncpy to strtomem

On Tue, Aug 08, 2023 at 09:17:35PM +0000, Justin Stitt wrote:
> Favor a more robust and less ambiguous interface over `strncpy`.
> 
> Use `strtomem` here since `console_buf` is not expected to be
> NUL-terminated.
> 
> Also mark char buffer as `__nonstring` as per Kees' suggestion here [1]
> 
> Link: https://github.com/KSPP/linux/issues/90 [1]
> Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Changes in v2:
> - Keep usage of ARRAY_SIZE (thanks Bill)
> - Remove unnecessary import (thanks Kees and Bill)
> - Link to v1: https://lore.kernel.org/r/20230807-arch-um-v1-1-86dbbfb59709@google.com
> ---
> Notes:
> I only build tested this patch.
> 
> I am also making an assumption that `console_buf` need not be
> NUL-terminated. Am I right in believing this?
> ---
>  arch/um/drivers/mconsole_kern.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
> index 5026e7b9adfe..d5fa9c8dcd18 100644
> --- a/arch/um/drivers/mconsole_kern.c
> +++ b/arch/um/drivers/mconsole_kern.c
> @@ -554,7 +554,7 @@ struct mconsole_output {
>  
>  static DEFINE_SPINLOCK(client_lock);
>  static LIST_HEAD(clients);
> -static char console_buf[MCONSOLE_MAX_DATA];
> +static char console_buf[MCONSOLE_MAX_DATA] __nonstring;
>  
>  static void console_write(struct console *console, const char *string,
>  			  unsigned int len)
> @@ -567,7 +567,7 @@ static void console_write(struct console *console, const char *string,
>  
>  	while (len > 0) {
>  		n = min((size_t) len, ARRAY_SIZE(console_buf));
> -		strncpy(console_buf, string, n);
> +		strtomem(console_buf, string);

See my reply to v1: I think this should just be memcpy(console_buf, string, n)

-Kees

>  		string += n;
>  		len -= n;
>  
> 
> ---
> base-commit: c1a515d3c0270628df8ae5f5118ba859b85464a2
> change-id: 20230807-arch-um-3ef24413427e
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook