linux-kernel - Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS

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Message-ID: <9c435a66-f8b1-5250-f3bf-cfada84d3d97@suse.com>
Date:   Fri, 5 May 2017 07:09:16 +0200
From:   Juergen Gross <jgross@...e.com>
To:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org
Cc:     boris.ostrovsky@...cle.com, hpa@...or.com, tglx@...utronix.de,
        mingo@...hat.com
Subject: Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when
 running under Xen

Any comments?


Juergen

On 27/04/17 07:01, Juergen Gross wrote:
> When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
> on AMD cpus.
> 
> This bug/feature bit is kind of special as it will be used very early
> when switching threads. Setting the bit and clearing it a little bit
> later leaves a critical window where things can go wrong. This time
> window has enlarged a little bit by using setup_clear_cpu_cap() instead
> of the hypervisor's set_cpu_features callback. It seems this larger
> window now makes it rather easy to hit the problem.
> 
> The proper solution is to never set the bit in case of Xen.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/kernel/cpu/amd.c   | 5 +++--
>  arch/x86/xen/enlighten_pv.c | 1 -
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c36140d788fe..b6da6e75e3a8 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -799,8 +799,9 @@ static void init_amd(struct cpuinfo_x86 *c)
>  		if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))
>  			set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);
>  
> -	/* AMD CPUs don't reset SS attributes on SYSRET */
> -	set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> +	/* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */
> +	if (!cpu_has(c, X86_FEATURE_XENPV))
> +		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
>  }
>  
>  #ifdef CONFIG_X86_32
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index a1af4f68278f..dcfd07faf1c3 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -290,7 +290,6 @@ static bool __init xen_check_xsave(void)
>  
>  static void __init xen_init_capabilities(void)
>  {
> -	setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS);
>  	setup_force_cpu_cap(X86_FEATURE_XENPV);
>  	setup_clear_cpu_cap(X86_FEATURE_DCA);
>  	setup_clear_cpu_cap(X86_FEATURE_APERFMPERF);
>