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

Tycho Andersen
2017-08-11T21:13:06Z

Hi Laura,

On Fri, Aug 11, 2017 at 10:25:14AM -0700, Laura Abbott wrote:
> On 08/09/2017 01:07 PM, Tycho Andersen wrote:
> > From: Juerg Haefliger <juerg.haefliger@hpe.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@canonical.com>
> > Tested-by: Tycho Andersen <tycho@docker.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?

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;

/*
* Take care not to create a writable alias for the

Cheers,

Tycho