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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Message-ID: <20160502211817.GA22492@test-lenovo>
Date:	Mon, 2 May 2016 14:18:17 -0700
From:	Yu-cheng Yu <yu-cheng.yu@...el.com>
To:	Dave Hansen <dave.hansen@...ux.intel.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 Mon, May 02, 2016 at 10:33:10AM -0700, Dave Hansen wrote:
> 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.

Before Linux gets into copy_fpstate_to_sigframe(),
current->thread.fpu.fpstate_active must be true.
For eagerfpu, fpregs_active() must also be true.
For lazyfpu, once we try to do FSAVE/FXSAVE/XSAVE,
fpregs_active() will become true as well.

We should have not based on boot_cpu_has(X86_FEATURE_XSAVES)
at all. 

Why don't we make it simple and always copy_fpregs_to_signal_frame()?
Or, only for the lazy case, i.e. !fpregs_active(), we do __copy_to_user().

Anyway, I think we can just replace may_copy_fpregs_to_sigframe() with
!fpregs_active().

Comments?