linux-kernel - Re: [kernel-hardening] [PATCH v5 06/10] arm64/mm: Disable section mappings if XPFO is enabled

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Date:   Fri, 11 Aug 2017 10:25:14 -0700
From:   Laura Abbott <labbott@...hat.com>
To:     Tycho Andersen <tycho@...ker.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        Juerg Haefliger <juerg.haefliger@....com>
Subject: Re: [kernel-hardening] [PATCH v5 06/10] arm64/mm: Disable section
 mappings if XPFO is enabled

On 08/09/2017 01:07 PM, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@....com>
> 
> XPFO (eXclusive Page Frame Ownership) doesn't support section mappings
> yet, so disable it if XPFO is turned on.
> 
> Signed-off-by: Juerg Haefliger <juerg.haefliger@...onical.com>
> Tested-by: Tycho Andersen <tycho@...ker.com>
> ---
>  arch/arm64/mm/mmu.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index f1eb15e0e864..38026b3ccb46 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -176,6 +176,18 @@ static void alloc_init_cont_pte(pmd_t *pmd, unsigned long addr,
>  	} while (addr = next, addr != end);
>  }
>  
> +static inline bool use_section_mapping(unsigned long addr, unsigned long next,
> +				unsigned long phys)
> +{
> +	if (IS_ENABLED(CONFIG_XPFO))
> +		return false;
> +
> +	if (((addr | next | phys) & ~SECTION_MASK) != 0)
> +		return false;
> +
> +	return true;
> +}
> +
>  static void init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
>  		     phys_addr_t phys, pgprot_t prot,
>  		     phys_addr_t (*pgtable_alloc)(void), int flags)
> @@ -190,7 +202,7 @@ static void init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
>  		next = pmd_addr_end(addr, end);
>  
>  		/* try section mapping first */
> -		if (((addr | next | phys) & ~SECTION_MASK) == 0 &&
> +		if (use_section_mapping(addr, next, phys) &&
>  		    (flags & NO_BLOCK_MAPPINGS) == 0) {
>  			pmd_set_huge(pmd, phys, prot);
>  
> 

There is already similar logic to disable section mappings for
debug_pagealloc at the start of map_mem, can you take advantage
of that?

Thanks,
Laura