linux-kernel - Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

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Date:   Mon, 5 Feb 2018 12:58:36 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     tglx@...utronix.de, Andi Kleen <ak@...ux.intel.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, luto@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, torvalds@...ux-foundation.org
Subject: Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall
 arguments for 64bit kernels


* Dan Williams <dan.j.williams@...el.com> wrote:

> +	/*
> +	 * Sanitize extra registers of values that a speculation attack
> +	 * might want to exploit. In the CONFIG_FRAME_POINTER=y case,
> +	 * the expectation is that %ebp will be clobbered before it
> +	 * could be used.
> +	 */
> +	.macro CLEAR_EXTRA_REGS_NOSPEC
> +	xorq %r15, %r15
> +	xorq %r14, %r14
> +	xorq %r13, %r13
> +	xorq %r12, %r12
> +	xorl %ebx, %ebx
> +#ifndef CONFIG_FRAME_POINTER
> +	xorl %ebp, %ebp
> +#endif

BTW., is there any reason behind the order of the clearing of these registers? 
This ordering seems rather random:

 - The canonical register order is: RBX, RBP, R12, R13, R14, R15, which is also 
   their push-order on the stack.

 - The CLEAR_EXTRA_REGS_NOSPEC order appears to be the reverse order (pop-order), 
   but with RBX and RBP reversed.

So since this is a 'push side' primitive I'd use the regular (push-) ordering 
instead:

	.macro CLEAR_EXTRA_REGS_NOSPEC
	xorl %ebx, %ebx
	xorl %ebp, %ebp
	xorq %r12, %r12
	xorq %r13, %r13
	xorq %r14, %r14
	xorq %r15, %r15

It obviously doesn't matter to correctness - only to readability.

There's also a (very) small micro-optimization argument in favor of the regular 
order: the earlier registers are more likely to be utilized by C functions, so the 
sooner we clear them, the less potential interaction these clearing instructions 
are going to have with any later use.

Thanks,

	Ingo