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:	Fri, 29 Apr 2016 15:43:39 -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 Fri, Apr 29, 2016 at 01:09:07PM -0700, Dave Hansen wrote:
> On 03/04/2016 10:12 AM, Yu-cheng Yu wrote:
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index 0fbf60c..09945f1 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -130,6 +130,45 @@ static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
> >  	return err;
> >  }
> >  
> > +static int may_copy_fpregs_to_sigframe(void)
> > +{
> > +	/*
> > +	 * In signal handling path, the kernel already checks if
> > +	 * FPU instructions have been used before it calls
> > +	 * copy_fpstate_to_sigframe(). We check this here again
> > +	 * to detect any potential mis-use and saving invalid
> > +	 * register values directly to a signal frame.
> > +	 */
> > +	WARN_ONCE(!current->thread.fpu.fpstate_active,
> > +		  "direct FPU save with no math use\n");
> 
> This is probably an OK check for this _particular_ context (since this
> context is all ready to copy_to_user() the fpu state).  But is it good
> generally?  Why couldn't you have a !fpstate_active thread that _was_
> fpregs_active?
> 
> Such a thread _could_ do a direct XSAVE with no issues.

But it won't come to this function unless fpstate_active is ture?