linux-kernel - Re: [PATCH v1 3/4] arm64: Fix the page leak in pud/pmd_set

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Date:   Wed, 14 Mar 2018 16:57:29 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     catalin.marinas@....com, will.deacon@....com, arnd@...db.de,
        ard.biesheuvel@...aro.org, marc.zyngier@....com,
        james.morse@....com, kristina.martsenko@....com,
        takahiro.akashi@...aro.org, gregkh@...uxfoundation.org,
        tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        akpm@...ux-foundation.org, toshi.kani@....com
Subject: Re: [PATCH v1 3/4] arm64: Fix the page leak in pud/pmd_set_huge



On 3/14/2018 4:23 PM, Mark Rutland wrote:
> On Wed, Mar 14, 2018 at 02:18:24PM +0530, Chintan Pandya wrote:
>> While setting huge page, we need to take care of
>> previously existing next level mapping. Since,
>> we are going to overrite previous mapping, the
>> only reference to next level page table will get
>> lost and the next level page table will be zombie,
>> occupying space forever. So, free it before
>> overriding.
> 
>> @@ -939,6 +940,9 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
>>   		return 0;
>>   
>>   	BUG_ON(phys & ~PUD_MASK);
>> +	if (pud_val(*pud) && !pud_huge(*pud))
>> +		free_page((unsigned long)__va(pud_val(*pud)));
>> +
>>   	set_pud(pudp, pfn_pud(__phys_to_pfn(phys), sect_prot));
>>   	return 1;
>>   }
>> @@ -953,6 +957,9 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot)
>>   		return 0;
>>   
>>   	BUG_ON(phys & ~PMD_MASK);
>> +	if (pmd_val(*pmd) && !pmd_huge(*pmd))
>> +		free_page((unsigned long)__va(pmd_val(*pmd)));
>> +
> 
> As Marc noted, (assuming the subsequent revert is applied) in both of
> these cases, these tables are still live, and thus it is not safe to
> free them.
> 
> Consider that immediately after freeing the pages, they may get
> re-allocated elsewhere, where they may be modified. If this happens
> before TLB invalidation, page table walks may allocate junk into TLBs,
> resulting in a number of problems.
Ah okay. What about this sequence,
1) I store old PMD/PUD values
2) Update the PMD/PUD with section mapping
3) Invalidate TLB
4) Then free the *leaked* page

> 
> It is *never* safe to free a live page table, therefore NAK to this
> patch.
> 
> Thanks,
> Mark.
> 

Chintan
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