linux-hardening - Re: [PATCH] kheaders: Use array declaration instead of char

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Date:   Fri, 3 Mar 2023 03:08:10 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jakub Kicinski <kuba@...nel.org>, stable@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kheaders: Use array declaration instead of char

On Thu, Mar 02, 2023 at 02:49:50PM -0800, Kees Cook wrote:
> Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination
> and source buffers. Defining kernel_headers_data as "char" would trip
> this check. Since these addresses are treated as byte arrays, define
> them as arrays (as done everywhere else).
> 
> This was seen with:
> 
>   $ cat /sys/kernel/kheaders.tar.xz >> /dev/null
> 
>   detected buffer overflow in memcpy
>   kernel BUG at lib/string_helpers.c:1027!
>   ...
>   RIP: 0010:fortify_panic+0xf/0x20
>   [...]
>   Call Trace:
>    <TASK>
>    ikheaders_read+0x45/0x50 [kheaders]
>    kernfs_fop_read_iter+0x1a4/0x2f0
>   ...
> 
> Reported-by: Jakub Kicinski <kuba@...nel.org>
> Link: https://lore.kernel.org/bpf/20230302112130.6e402a98@kernel.org/
> Tested-by: Jakub Kicinski <kuba@...nel.org>

Acked-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel


> Fixes: 43d8ce9d65a5 ("Provide in-kernel headers to make extending kernel easier")
> Cc: Joel Fernandes (Google) <joel@...lfernandes.org>
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  kernel/kheaders.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/kheaders.c b/kernel/kheaders.c
> index 8f69772af77b..42163c9e94e5 100644
> --- a/kernel/kheaders.c
> +++ b/kernel/kheaders.c
> @@ -26,15 +26,15 @@ asm (
>  "	.popsection				\n"
>  );
>  
> -extern char kernel_headers_data;
> -extern char kernel_headers_data_end;
> +extern char kernel_headers_data[];
> +extern char kernel_headers_data_end[];
>  
>  static ssize_t
>  ikheaders_read(struct file *file,  struct kobject *kobj,
>  	       struct bin_attribute *bin_attr,
>  	       char *buf, loff_t off, size_t len)
>  {
> -	memcpy(buf, &kernel_headers_data + off, len);
> +	memcpy(buf, &kernel_headers_data[off], len);
>  	return len;
>  }
>  
> @@ -48,8 +48,8 @@ static struct bin_attribute kheaders_attr __ro_after_init = {
>  
>  static int __init ikheaders_init(void)
>  {
> -	kheaders_attr.size = (&kernel_headers_data_end -
> -			      &kernel_headers_data);
> +	kheaders_attr.size = (kernel_headers_data_end -
> +			      kernel_headers_data);
>  	return sysfs_create_bin_file(kernel_kobj, &kheaders_attr);
>  }
>  
> -- 
> 2.34.1
>