linux-kernel - Re: question about save_xstate_sig()

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Message-ID: <54C2B80B.8050501@zytor.com>
Date:	Fri, 23 Jan 2015 13:07:23 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

On 01/23/2015 11:34 AM, Rik van Riel wrote:
> While working on a patch series to defer FPU state loading until
> kernel -> user space transition, and be more lazy with FPU state
> while in the kernel, I came across this code in save_xstate_sig().
> 
> Not only is this broken with my new code, but it looks like it may
> be broken with the current code, too...
> 
> Specifically, save_user_xstate() may page fault and sleep. After
> returning from the page fault, there is no guarantee that the
> FPU state will be restored into the CPU, when the system is not
> running with eager fpu mode.
> 
> In that case, what prevents us from saving random FPU register state
> to the user's stack frame?  Potentially state containing data from
> other programs...
> 

If the FPU state is not current, we'll have CR0.TS = 1 and the XSAVE
will cause an #NM exception, which will cause the FPU state to be
swapped in.

	-hpa


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