linux-kernel - Re: [PATCH] x86/fpu/xstate: Clear uninitialized xstate areas in core dump

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Date:   Thu, 07 May 2020 11:41:43 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     sam <sunhaoyl@...look.com>, Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/fpu/xstate: Clear uninitialized xstate areas in
 core dump

On Thu, 2020-05-07 at 20:22 +0200, Thomas Gleixner wrote:
> Yu-cheng Yu <yu-cheng.yu@...el.com> writes:
> > @@ -983,6 +983,7 @@ int copy_xstate_to_kernel(void *kbuf, struct xregs_state *xsave, unsigned int of
> >  {
> >  	unsigned int offset, size;
> >  	struct xstate_header header;
> > +	int last_off;
> >  	int i;
> >  
> >  	/*
> > @@ -1006,7 +1007,17 @@ int copy_xstate_to_kernel(void *kbuf, struct xregs_state *xsave, unsigned int of
> >  
> >  	__copy_xstate_to_kernel(kbuf, &header, offset, size, size_total);
> >  
> > +	last_off = 0;
> > +
> >  	for (i = 0; i < XFEATURE_MAX; i++) {
> > +		/*
> > +		 * Clear uninitialized area before XSAVE header.
> > +		 */
> > +		if (i == FIRST_EXTENDED_XFEATURE) {
> > +			memset(kbuf + last_off, 0, XSAVE_HDR_OFFSET - last_off);
> > +			last_off = XSAVE_HDR_OFFSET + XSAVE_HDR_SIZE;
> > +		}
> > +
> >  		/*
> >  		 * Copy only in-use xstates:
> >  		 */
> > @@ -1020,11 +1031,16 @@ int copy_xstate_to_kernel(void *kbuf, struct xregs_state *xsave, unsigned int of
> >  			if (offset + size > size_total)
> >  				break;
> >  
> > +			memset(kbuf + last_off, 0, offset - last_off);
> > +			last_off = offset + size;
> > +
> >  			__copy_xstate_to_kernel(kbuf, src, offset, size, size_total);
> >  		}
> >  
> >  	}
> >  
> > +	memset(kbuf + last_off, 0, size_total - last_off);
> 
> Why doing all this partial zeroing? There is absolutely no point.
> 
> Either the caller clears the buffer or this function clears it right at
> the beginning with:
> 
>     memset(kbuf, 0, min(size_total, XSAVE_MAX_SIZE));

I was concerned that the XSAVES buffer can be large, but this is not in a
performance-critical path.  Yes, clear it in the beginning is simpler.

Yu-cheng