linux-kernel - Re: [PATCH] x86/fpu/xstate: Fix xcomp

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Date:   Tue, 24 Jan 2017 09:09:57 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Joakim Tjernlund <Joakim.Tjernlund@...inera.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, haokexin@...il.com
Subject: Re: [PATCH] x86/fpu/xstate: Fix xcomp_bv in XSAVES header


* Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:

> The compacted-format XSAVES area is determined at boot time and
> never changed after.  The field xsave.header.xcomp_bv indicates
> which components are in the fixed XSAVES format.
> 
> In fpstate_init() we did not set xcomp_bv to reflect the XSAVES
> format since at the time there is no valid data.
> 
> However, after we do copy_init_fpstate_to_fpregs() in fpu__clear(),
> as in commit: b22cbe404a9cc3c7949e380fa1861e31934c8978, and when
> __fpu_restore_sig() does fpu__restore() for a COMPAT-mode app,
> a #GP occurs.  This can be easily triggered by doing valgrind on
> a COMPAT-mode "Hello World," as reported by Joakim Tjernlund and
> others:
> 
> 	https://bugzilla.kernel.org/show_bug.cgi?id=190061
> 
> Fix it by setting xcomp_bv correctly.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Reported-by: Joakim Tjernlund <Joakim.Tjernlund@...inera.com>
> ---
>  arch/x86/kernel/fpu/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index c289e2f..e540dc1 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -9,6 +9,7 @@
>  #include <asm/fpu/regset.h>
>  #include <asm/fpu/signal.h>
>  #include <asm/fpu/types.h>
> +#include <asm/fpu/xstate.h>
>  #include <asm/traps.h>
>  
>  #include <linux/hardirq.h>
> @@ -235,7 +236,8 @@ void fpstate_init(union fpregs_state *state)
>  	 * it will #GP. Make sure it is replaced after the memset().
>  	 */
>  	if (static_cpu_has(X86_FEATURE_XSAVES))
> -		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
> +		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> +					       xfeatures_mask;

Ok, I have applied this - but it would be cleaner to go one step further and add a 
fpstate_init_xstate() method that does this in xstate.c and hides the details from 
arch/x86/kernel/fpu/core.c.

Similar to how the FX-state initialization is done today:

>  	if (static_cpu_has(X86_FEATURE_FXSR))
>  		fpstate_init_fxstate(&state->fxsave);

Thanks,

	Ingo