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

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Message-ID: <alpine.DEB.2.21.1906252100290.32342@nanos.tec.linutronix.de>
Date:   Tue, 25 Jun 2019 21:04:39 +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 Thu, 20 Jun 2019, Kirill A. Shutemov wrote:
> @@ -190,18 +190,18 @@ unsigned long __head __startup_64(unsigned long physaddr,
>  		pgd[i + 0] = (pgdval_t)p4d + pgtable_flags;
>  		pgd[i + 1] = (pgdval_t)p4d + pgtable_flags;
>  
> -		i = (physaddr >> P4D_SHIFT) % PTRS_PER_P4D;
> -		p4d[i + 0] = (pgdval_t)pud + pgtable_flags;
> -		p4d[i + 1] = (pgdval_t)pud + pgtable_flags;
> +		i = physaddr >> P4D_SHIFT;
> +		p4d[(i + 0) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
> +		p4d[(i + 1) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
>  	} else {
>  		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
>  		pgd[i + 0] = (pgdval_t)pud + pgtable_flags;
>  		pgd[i + 1] = (pgdval_t)pud + pgtable_flags;
>  	}
>  
> -	i = (physaddr >> PUD_SHIFT) % PTRS_PER_PUD;
> -	pud[i + 0] = (pudval_t)pmd + pgtable_flags;
> -	pud[i + 1] = (pudval_t)pmd + pgtable_flags;
> +	i = physaddr >> PUD_SHIFT;
> +	pud[(i + 0) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
> +	pud[(i + 1) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
>  
>  	pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
>  	/* Filter out unsupported __PAGE_KERNEL_* bits: */
> @@ -211,8 +211,8 @@ unsigned long __head __startup_64(unsigned long physaddr,
>  	pmd_entry +=  physaddr;
>  
>  	for (i = 0; i < DIV_ROUND_UP(_end - _text, PMD_SIZE); i++) {
> -		int idx = i + (physaddr >> PMD_SHIFT) % PTRS_PER_PMD;
> -		pmd[idx] = pmd_entry + i * PMD_SIZE;
> +		int idx = i + (physaddr >> PMD_SHIFT);;

double semicolon

> +		pmd[idx % PTRS_PER_PMD] = pmd_entry + i * PMD_SIZE;

This part is functionally equivivalent. So what's the value of this change?

Thanks,

	tglx