linux-kernel - Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments

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Date:	Tue, 17 Nov 2015 14:59:24 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	unlisted-recipients:; (no To-header on input)
CC:	Xen-devel <xen-devel@...ts.xen.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<lguest@...ts.ozlabs.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments

Ping?

None of the discussion on this thread altered the contents of this
patch, and the bug is still present.

~Andrew

On 03/06/15 10:31, Andrew Cooper wrote:
> There appears to be no formal statement of what pv_irq_ops.save_fl() is
> supposed to return precisely.  Native returns the full flags, while lguest and
> Xen only return the Interrupt Flag, and both have comments by the
> implementations stating that only the Interrupt Flag is looked at.  This may
> have been true when initially implemented, but no longer is.
>
> To make matters worse, the Xen PVOP leaves the upper bits undefined, making
> the BUG_ON() undefined behaviour.  Experimentally, this now trips for 32bit PV
> guests on Broadwell hardware.  The BUG_ON() is consistent for an individual
> build, but not consistent for all builds.  It has also been a sitting timebomb
> since SMAP support was introduced.
>
> Use native_save_fl() instead, which will obtain an accurate view of the AC
> flag.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
> Reviewed-by: David Vrabel <david.vrabel@...rix.com>
> Tested-by: Rusty Russell <rusty@...tcorp.com.au>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: Ingo Molnar <mingo@...hat.com>
> CC: H. Peter Anvin <hpa@...or.com>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> CC: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> CC: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> CC: xen-devel <xen-devel@...ts.xen.org>
> CC: lguest@...ts.ozlabs.org
> CC: stable@...r.kernel.org
> ---
>  arch/x86/kernel/cpu/common.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index a62cf04..4f2fded 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -291,10 +291,9 @@ __setup("nosmap", setup_disable_smap);
>  
>  static __always_inline void setup_smap(struct cpuinfo_x86 *c)
>  {
> -	unsigned long eflags;
> +	unsigned long eflags = native_save_fl();
>  
>  	/* This should have been cleared long ago */
> -	raw_local_save_flags(eflags);
>  	BUG_ON(eflags & X86_EFLAGS_AC);
>  
>  	if (cpu_has(c, X86_FEATURE_SMAP)) {

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