On Wed, Aug 6, 2014 at 10:21 AM, Nicolas Pitre [off-list ref] wrote:

On Wed, 6 Aug 2014, Kees Cook wrote:

quoted

With the original code, there is (I think) a page table entry for the
fixmap range. For the latter, there isn't. I see a NULL pgd entry
fault when trying to use it, and noticed that this only exists under
highmem:

static void __init kmap_init(void)
{
#ifdef CONFIG_HIGHMEM
        pkmap_page_table = early_pte_alloc(pmd_off_k(PKMAP_BASE),
                PKMAP_BASE, _PAGE_KERNEL_TABLE);

        fixmap_page_table = early_pte_alloc(pmd_off_k(FIXADDR_START),
                FIXADDR_START, _PAGE_KERNEL_TABLE);
#endif
}

The above is wrong.  The fixmap PTEs must be allocated unconditionally
irrespective of highmem.  So the #endif should be moved up by 3 lines.

That's what I was thinking. I tried this, and things are still weird,
though I think I'm narrowing it down. I made the early_pte_alloc
happen, but after boot it doesn't show up in
/sys/kernel/debug/kernel_page_tables. When I attempt a fixmap text
poke, I get this out of dmesg (lkdtm is reporting the address returned
from the patch mapping):

lkdtm: 80421ac0 mapped RW via ffdefac0
Unable to handle kernel paging request at virtual address ffdefac0
pgd = 9e888000
[ffdefac0] *pgd=9fffc811, *pte=00000000, *ppte=00000000
Internal error: Oops: 807 [#1] SMP ARM

The target address (0xffdefac0) is correct; it's in the range set aside:

#define FIXADDR_START           0xffc00000UL
#define FIXADDR_END             0xffe00000UL
#define FIXADDR_TOP             (FIXADDR_END - PAGE_SIZE)

enum fixed_addresses {
        /* Support 16 CPUs for kmap as the first region of fixmap entries. */
        FIX_KMAP_BEGIN,
        FIX_KMAP_END = 15,

        /* Support writing RO kernel text via kprobes, jump labels, etc. */
        FIX_TEXT_POKE0,
        FIX_TEXT_POKE1,

        __end_of_fixed_addresses
};

But the resulting PTE is in totally the wrong location as reported by
/sys/kernel/debug/kernel_page_tables:

---[ Fixmap Area ]---
0xfffef000-0xffff0000           4K     RW NX SHD MEM/CACHED/WBWA

I continue to dig...

-Kees

-- 
Kees Cook
Chrome OS Security