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

[Tycho Andersen]

On Sat, Aug 12, 2017 at 12:17:34PM +0100, Mark Rutland wrote:
> 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.

Yes, thanks!

Tycho