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 13:09:07 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:	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 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.