linux-kernel - Re: [PATCH] security/loadpin: Replace "kernel_read_file_str[j]" with function "kernel_read_file_id

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Message-ID: <202103041547.2D77671E92@keescook>
Date:   Thu, 4 Mar 2021 15:50:29 -0800
From:   Kees Cook <keescook@...omium.org>
To:     zhaojiele <unclexiaole@...il.com>
Cc:     jmorris@...ei.org, serge@...lyn.com,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] security/loadpin: Replace "kernel_read_file_str[j]" with
 function                   "kernel_read_file_id_str(j)".

On Thu, Mar 04, 2021 at 08:36:38AM +0000, zhaojiele wrote:
> Actually Linux kernel already provide function "kernel_read_file_id_str()"
> for secure access in "kernel_read_file.h".And, in "parse_exclude()"
> function, there is no need for
> "BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
> ARRAY_SIZE(ignore_read_file_id))"
> when access array by functon "kernel_read_file_id_str(j)".
> 
> Signed-off-by: zhaojiele <unclexiaole@...il.com>
> ---
>  security/loadpin/loadpin.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> index b12f7d986b1e..3e8bdcd06600 100644
> --- a/security/loadpin/loadpin.c
> +++ b/security/loadpin/loadpin.c
> @@ -210,8 +210,6 @@ static void __init parse_exclude(void)
>  	 */
>  	BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
>  		     ARRAY_SIZE(ignore_read_file_id));
> -	BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
> -		     ARRAY_SIZE(ignore_read_file_id));

This should stay to make sure kernel_read_file_str doesn't diverge from
the other two. However, maybe it should be tightened to:

	BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) - 1 ==
		     ARRAY_SIZE(ignore_read_file_id));

>  
>  	for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
>  		cur = exclude_read_files[i];
> @@ -221,9 +219,9 @@ static void __init parse_exclude(void)
>  			continue;
>  
>  		for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
> -			if (strcmp(cur, kernel_read_file_str[j]) == 0) {
> +			if (strcmp(cur, kernel_read_file_id_str(j)) == 0) {
>  				pr_info("excluding: %s\n",
> -					kernel_read_file_str[j]);
> +					kernel_read_file_id_str(j));
>  				ignore_read_file_id[j] = 1;
>  				/*
>  				 * Can not break, because one read_file_str

I feel funny about making these into function calls when we've already
validated the index, but yeah, that would be fine. Can you send a v2
with the earlier suggestion addressed?

Thanks!

-Kees

-- 
Kees Cook