linux-kernel - Re: [PATCH urgent] x86, asm: Disable opportunistic SYSRET if regs->flags has TF set

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Date:	Wed, 01 Apr 2015 23:18:16 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org
CC:	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH urgent] x86, asm: Disable opportunistic SYSRET if regs->flags
 has TF set

On 04/01/2015 09:25 PM, Andy Lutomirski wrote:
> Fix it by using IRET to restore TF.  Since it's late, I'm keeping
> this minimal and keeping "testq" instead of switching to "testl".

Changing to "testl" here wins nothing. Since r11 is used,
REX prefix will be encoded anyway.

>  
> -	testq $X86_EFLAGS_RF,%r11	/* sysret can't restore RF */
> +	/*
> +	 * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
> +	 * restoring TF results in a trap from userspace immediately after
> +	 * SYSRET.

>                  This would cause an infinite loop whenever #DB happens
> +	 * with register state that satisfies the opportunistic SYSRET
> +	 * conditions.
> +	 */

I propose to just show an example of the affected code:

>                  This can cause an infinite loop. Example:
>	 * asm volatile("movq $1f,%rcx\n\t"
>	 *		"pushfq\n\t"
>	 *		"popq %r11\n\t"
>	 *		"nop\n\t"
>	 *		"1:");
>	 * The above example would get stuck at "1:".
>	 */



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