linux-kernel - Re: [PATCH] x86/boot/64: Fix crash if kernel images crosses page table boundary

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Message-ID: <alpine.DEB.2.21.1906252155430.32342@nanos.tec.linutronix.de>
Date:   Tue, 25 Jun 2019 21:56:56 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] x86/boot/64: Fix crash if kernel images crosses page
 table boundary

On Tue, 25 Jun 2019, Kirill A. Shutemov wrote:
> On Tue, Jun 25, 2019 at 09:04:39PM +0200, Thomas Gleixner wrote:
> 
> > > +		pmd[idx % PTRS_PER_PMD] = pmd_entry + i * PMD_SIZE;
> > 
> > This part is functionally equivivalent. So what's the value of this change?
> 
> Precedence of operators were broken
> 
> 	idx =  i + (physaddr >> PMD_SHIFT) % PTRS_PER_PMD;
> 
> reads by compiler as
> 
> 	idx = i + ((physaddr >> PMD_SHIFT) % PTRS_PER_PMD);
> 
> not as
> 
> 	idx =  (i + (physaddr >> PMD_SHIFT)) % PTRS_PER_PMD;
> 
> Therefore 'idx' can become >= PTRS_PER_PMD.

Indeed. Please mention it in the change log. I did not spot it right away.

Thanks,

	tglx