Messages in this thread | | | Date | Wed, 6 Aug 2014 10:28:37 -0700 | Subject | Re: [PATCH v3 2/2] ARM : change fixmap mapping region to support 32 CPUs | From | Kees Cook <> |
| |
On Wed, Aug 6, 2014 at 10:21 AM, Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > On Wed, 6 Aug 2014, Kees Cook wrote: > >> 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
| |