On Wed, Aug 6, 2014 at 3:16 PM, Russell King - ARM Linux
[off-list ref] wrote:

On Wed, Aug 06, 2014 at 10:28:37AM -0700, Kees Cook wrote:

quoted

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...

This sounds like you have an erroneous merge conflict resolution.  You
seem to be poking into TOP_PTE rather than fixmap_page_table.

Yeah, agreed. This did turn out to be the issue, so I reworked the
set_fixmap routine to aim at the correct pte. The result seems to
work, so I after retesting everything, I sent the series out. So
close! :)

-Kees

-- 
Kees Cook
Chrome OS Security