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 17:36:48 -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 04/29/2016 03:43 PM, Yu-cheng Yu wrote:
> On Fri, Apr 29, 2016 at 01:09:07PM -0700, Dave Hansen wrote:
>> On 03/04/2016 10:12 AM, Yu-cheng Yu wrote:
>>> +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?

If may_copy_fpregs_to_sigframe() were called from a slightly different
context, or if we change the call-site, what breaks?

In other words. if we can still "may_copy_fpregs_to_sigframe()" no
matter the state of fpu.fpstate_active, then I don't think we should be
checking it in may_copy_fpregs_to_sigframe().