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Subject: Re: [PATCH v2 RESEND] m68k/kernel: array out of bound access in
 process_uboot_commandline
To:     Hangyu Hua <hbh25y@gmail.com>, geert@linux-m68k.org
Cc:     schwab@linux-m68k.org, linux-m68k@lists.linux-m68k.org,
        linux-kernel@vger.kernel.org
References: <20220113015854.9326-1-hbh25y@gmail.com>
From:   Greg Ungerer <gerg@linux-m68k.org>
Message-ID: <95d91f5f-c5e1-0750-ebb9-b6839aecdc7c@linux-m68k.org>
Date:   Mon, 17 Jan 2022 14:03:45 +1000
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Hi Hangyu,

On 13/1/22 11:58 am, Hangyu Hua wrote:
> When the size of commandp >= size, array out of bound write occurs because
> len == 0.
> 
> Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
> ---
>   arch/m68k/kernel/uboot.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/m68k/kernel/uboot.c b/arch/m68k/kernel/uboot.c
> index 928dbd33fc4a..63eaf3c3ddcd 100644
> --- a/arch/m68k/kernel/uboot.c
> +++ b/arch/m68k/kernel/uboot.c
> @@ -101,5 +101,6 @@ __init void process_uboot_commandline(char *commandp, int size)
>   	}
>   
>   	parse_uboot_commandline(commandp, len);
> -	commandp[len - 1] = 0;
> +	if (len > 0)
> +		commandp[len - 1] = 0;
>   }
> 

I am not convinced this is wrong for the reason you think it is.
Looking at the code in its entirety:

__init void process_uboot_commandline(char *commandp, int size)
{
         int len, n;

         n = strnlen(commandp, size);
         commandp += n;
         len = size - n;
         if (len) {
                 /* Add the whitespace separator */
                 *commandp++ = ' ';
                 len--;
         }

         parse_uboot_commandline(commandp, len);
         commandp[len - 1] = 0;
}


"commandp" is moved based on the return of the strnlen(). So in the
case of commandp actually being full of valid characters (so n == size,
and thus len == 0) the commandp technically points outside of its
real size at that point. But "command[[len - 1]" would actually be
pointing to the last char in the original commandp array (so the original
commandp[size - 1]). Well at least if you are happy with the use of
negative array indexes.

Clearly this could be structured better. There is no point in calling
parse_uboot_commandline() if len == 0, or even if len == 1, since you
cannot add anymore to the command line, it is full.

Regards
Greg