linux-kernel - RE: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use user_xstate

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Date:	Tue, 28 Apr 2015 22:11:14 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	"Hansen, Dave" <dave.hansen@...el.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Williamson, Glenn P" <glenn.p.williamson@...el.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use
 user_xstate_size for xstate size in signal context

> From: Hansen, Dave
> Sent: Tuesday, April 28, 2015 3:09 PM
> To: Yu, Fenghua; H. Peter Anvin; Ingo Molnar; Thomas Gleixner; Mallick, Asit
> K; Williamson, Glenn P
> Cc: linux-kernel; x86
> Subject: Re: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use
> user_xstate_size for xstate size in signal context
> 
> On 04/21/2015 09:51 PM, Fenghua Yu wrote:
> > +	/*
> > +	 * Clear xcomp_bv[63] in user's xsave area header to indicate
> > +	 * buf_fx is in standard format.
> > +	 */
> > +	xcomp_bv = xsave->xsave_hdr.xcomp_bv;
> > +	user_xsave = buf_fx;
> > +	xcomp_bv &= ~((u64)1 << 63);
> 
> I ran in to another bug.  xrestor_user() is hitting a #GP with these values:
> 
> [    6.258743] xrestore_user() tmp: ffff88003f813000
> [    6.261122] 	     XCR0: 000000000000001f
> [    6.261868] 	xstate_bv: 0000000000000003
> [    6.262613] 	 xcomp_bv: 000000000000001f
> 
> I think it is because bit 63 is clear in xcomp_bv, but there are other bits set in
> there.
> 
> I think the above needs to just do:
> 
> 	/*
> 	 * We are uncompacting the state for the user buffer.  We need
> 	 * to clear out the xcomp_bv field entirely.  The uncompacted
> 	 * form of xsave/xrstor treats this field as reserved.
> 	 */
> 	if (__put_user(0, &user_xsave->xsave_hdr.xcomp_bv))
> 		return -1;

Yes, that makes sense. I will add this in v3 patchset.

Thanks.

-Fenghua
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