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 10:35:55 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Chintan Pandya <cpandya@...eaurora.org>, catalin.marinas@....com,
        will.deacon@....com, arnd@...db.de
Cc:     mark.rutland@....com, ard.biesheuvel@...aro.org,
        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 14/03/18 08:48, 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.
> 
> Signed-off-by: Chintan Pandya <cpandya@...eaurora.org>
> ---
>  arch/arm64/mm/mmu.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 8c704f1..c0df264 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -32,7 +32,7 @@
>  #include <linux/io.h>
>  #include <linux/mm.h>
>  #include <linux/vmalloc.h>
> -
> +#include <linux/hugetlb.h>
>  #include <asm/barrier.h>
>  #include <asm/cputype.h>
>  #include <asm/fixmap.h>
> @@ -45,6 +45,7 @@
>  #include <asm/memblock.h>
>  #include <asm/mmu_context.h>
>  #include <asm/ptdump.h>
> +#include <asm/page.h>
>  
>  #define NO_BLOCK_MAPPINGS	BIT(0)
>  #define NO_CONT_MAPPINGS	BIT(1)
> @@ -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)));
> +

This is absolutely scary. Isn't this page still referenced in the page
tables (assuming patch 4 has been applied too)?

>  	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)));
> +
>  	set_pmd(pmdp, pfn_pmd(__phys_to_pfn(phys), sect_prot));
>  	return 1;
>  }
> 

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...