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

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Message-ID: <20170812111733.GA16374@remoulade>
Date:   Sat, 12 Aug 2017 12:17:34 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     Laura Abbott <labbott@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>
Subject: Re: [kernel-hardening] [PATCH v5 06/10] arm64/mm: Disable section
 mappings if XPFO is enabled

Hi,

On Fri, Aug 11, 2017 at 03:13:02PM -0600, Tycho Andersen wrote:
> On Fri, Aug 11, 2017 at 10:25:14AM -0700, Laura Abbott wrote:
> > On 08/09/2017 01:07 PM, Tycho Andersen wrote:
> > > @@ -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?
> 
> You're suggesting something like this instead? Seems to work fine.
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 38026b3ccb46..3b2c17bbbf12 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -434,6 +434,8 @@ static void __init map_mem(pgd_t *pgd)
>  
>  	if (debug_pagealloc_enabled())
>  		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> +	if (IS_ENABLED(CONFIG_XPFO))
> +		flags |= NO_BLOCK_MAPPINGS;
>  

IIUC, XPFO carves out individual pages just like DEBUG_PAGEALLOC, so you'll
also need NO_CONT_MAPPINGS.

Thanks
Mark.