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Date: Mon, 12 Jan 2015 15:47:16 -0800
From: Dave Hansen <dave.hansen@linux.intel.com>
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To: Andy Lutomirski <luto@amacapital.net>, x86@kernel.org,
        linux-kernel@vger.kernel.org
CC: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Subject: Re: [PATCH 3.19 v2 2/3] x86, mpx: Short-circuit the instruction decoder
 for unexpected opcodes
References: <cover.1421103159.git.luto@amacapital.net> <6cb492e6037780a61b973cc9e40050d43452ed63.1421103159.git.luto@amacapital.net>
In-Reply-To: <6cb492e6037780a61b973cc9e40050d43452ed63.1421103159.git.luto@amacapital.net>
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Couple of typos...

On 01/12/2015 03:04 PM, Andy Lutomirski wrote:
> +	/*
> +	 * We only _really_ need to decode bndcl/bndcn/bndcu
> +	 * Error out on anything else.  Check this before decoding the
> +	 * instruction to reduce our exposure to intentionally bad code
> +	 * to some extent.  Note that this shortcut cat incorrectly return

"...can incorrectly return"

> +	 * -EINVAL instead of -EFAULT under some circumstances.  This
> +	 * discrepency has no effect.
> +	 */

	^^ discrepancy


> +	if (nr_copied < 2)
> +		goto bad_opcode;
> +	if (buf[0] != 0x0f)
> +		goto bad_opcode;
> +	if (buf[1] != 0x1a && buf[1] != 0x1b)
> +		goto bad_opcode;
...
> -	/*
> -	 * We only _really_ need to decode bndcl/bndcn/bndcu
> -	 * Error out on anything else.
> -	 */
> -	if (insn->opcode.bytes[0] != 0x0f)
> -		goto bad_opcode;
> -	if ((insn->opcode.bytes[1] != 0x1a) &&
> -	    (insn->opcode.bytes[1] != 0x1b))
> -		goto bad_opcode;

Otherwise, this looks OK to me.  Have you tested this at all?  I know
you don't have any MPX hardware, but you can still hack something in to
point the instruction decoder at an MPX binary.