linux-kernel - Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Wed, 01 Oct 2014 08:22:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>
CC:	Sebastian Lackner <sebastian@...-team.de>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

On 09/30/2014 09:51 PM, Andy Lutomirski wrote:
> 
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 4299eb05023c..44d1dd371454 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target)
>  1:	movl	(%rbp),%ebp
>  	_ASM_EXTABLE(1b,ia32_badarg)
>  	ASM_CLAC
> +
> +	/*
> +	 * Sysenter doesn't filter flags, so we need to clear NT
> +	 * ourselves.  To save a few cycles, we can check whether
> +	 * NT was set instead of doing an unconditional popfq.
> +	 */
> +	testl $X86_EFLAGS_NT,EFLAGS(%rsp)	/* saved EFLAGS match cpu */
> +	jz 1f
> +	pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
> +	popfq_cfi
> +1:
> +

I'm wondering if it would be easier to just remove ASM_CLAC and do this
unconditionally.  On SMAP-enabled hardware then that gives us back some
of the cycles, may make the branch unnecessary.

	-hpa


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