Re: [PATCH v3 2/2] ARM : change fixmap mapping region to support 32 CPUs

[Kees Cook]

On Wed, Aug 6, 2014 at 3:16 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Aug 06, 2014 at 10:28:37AM -0700, Kees Cook wrote:
>> 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
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