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

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Date:   Tue, 25 Jun 2019 22:30:53 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Jun 25, 2019 at 09:04:39PM +0200, Thomas Gleixner wrote:
> 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

Will fix.

> > +		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.

-- 
 Kirill A. Shutemov