linux-kernel - Re: [RFC PATCH 2/2] x86/cmdline: Use strscpy to initialize boot_command

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Message-ID: <f5a29e70-7d11-16ec-8d72-ed71da4124c1@infradead.org>
Date:   Sat, 5 Sep 2020 15:59:04 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>, x86@...nel.org,
        kasan-dev@...glegroups.com
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] x86/cmdline: Use strscpy to initialize
 boot_command_line

On 9/5/20 3:23 PM, Arvind Sankar wrote:
> The x86 boot protocol requires the kernel command line to be a
> NUL-terminated string of length at most COMMAND_LINE_SIZE (including the
> terminating NUL). In case the bootloader messed up and the command line
> is too long (hence not NUL-terminated), use strscpy to copy the command
> line into boot_command_line. This ensures that boot_command_line is
> NUL-terminated, and it also avoids accessing beyond the actual end of
> the command line if it was properly NUL-terminated.
> 
> Note that setup_arch() will already force command_line to be
> NUL-terminated by using strlcpy(), as well as boot_command_line if a
> builtin command line is configured. If boot_command_line was not
> initially NUL-terminated, the strlen() inside of strlcpy()/strlcat()
> will run beyond boot_command_line, but this is almost certainly
> harmless in practice.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

Hi,
Just for my enlightenment, what would be wrong with:

(which is done in arch/m68/kernel/setup_no.c)

> ---
>  arch/x86/kernel/head64.c  |  2 +-
>  arch/x86/kernel/head_32.S | 11 +++++------
>  2 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index cbb71c1b574f..740dd05b9462 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -410,7 +410,7 @@ static void __init copy_bootdata(char *real_mode_data)
>  	cmd_line_ptr = get_cmd_line_ptr();
>  	if (cmd_line_ptr) {
>  		command_line = __va(cmd_line_ptr);
> 		memcpy(boot_command_line, command_line, COMMAND_LINE_SIZE);
> +		boot_command_line[COMMAND_LINE_SIZE - 1] = 0;
>  	}
>  
>  	/*


thanks.
-- 
~Randy