linux-kernel - Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows correct xstates copy from kernel to user directly

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Date:	Mon, 2 May 2016 10:33:10 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
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>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly

On 05/02/2016 10:19 AM, Yu-cheng Yu wrote:
> On Mon, May 02, 2016 at 09:43:47AM -0700, Dave Hansen wrote:
>>> If (fpu.fpstate_active == 0), then the task does not use FPU; we don't
>>> want to save these registers, right?  
>>
>> No.  It's possible to have fpstate_active=0 while fpregs_active=1.  Such
>> a task uses the FPU, but just hasn't done an XSAVE* to save the register
>> content to the fpstate buffer.
>>
>> Note, this is just theoretical, and does not happen in this particular
>> call path today.
> 
> What about...
> 
> static int may_copy_fpregs_to_sigframe(void)
> {
> 	if (fpregs_active())
> 		return 1;
> 
> 	WARN_ONCE(!current->thread.fpu.fpstate_active,
> 		  "direct FPU save with no math use\n");
> 
> 	if (boot_cpu_has(X86_FEATURE_XSAVES))
> 		return 1;
> 
> 	return 0;
> } 

I don't think that changes anything.  We still have a check in there
that has no purpose.  You've changed the ordering so that the specific
example that I pointed out no longer triggers it.  But, the underlying
issue remains.