With the following commit:

  4bc9f92e ("x86/efi-bgrt: Use efi_mem_reserve() to avoid copying image data")

...  efi_bgrt_init() calls into the memblock allocator through
efi_mem_reserve() => efi_arch_mem_reserve() *after* mm_init() has been called.

Indeed, KASAN reports a bad read access later on in efi_free_boot_services():

  BUG: KASAN: use-after-free in efi_free_boot_services+0xae/0x24c
            at addr ffff88022de12740
  Read of size 4 by task swapper/0/0
  page:ffffea0008b78480 count:0 mapcount:-127
  mapping:          (null) index:0x1 flags: 0x5fff8000000000()
  [...]
  Call Trace:
   dump_stack+0x68/0x9f
   kasan_report_error+0x4c8/0x500
   kasan_report+0x58/0x60
   __asan_load4+0x61/0x80
   efi_free_boot_services+0xae/0x24c
   start_kernel+0x527/0x562
   x86_64_start_reservations+0x24/0x26
   x86_64_start_kernel+0x157/0x17a
   start_cpu+0x5/0x14

The instruction at the given address is the first read from the memmap's
memory, i.e. the read of md->type in efi_free_boot_services().

Note that the writes earlier in efi_arch_mem_reserve() don't splat because
they're done through early_memremap()ed addresses.

So, after memblock is gone, allocations should be done through the "normal"
page allocator. Introduce a helper, efi_memmap_alloc() for this. Use
it from efi_arch_mem_reserve(), efi_free_boot_services() and, for the sake
of consistency, from efi_fake_memmap() as well.

Note that for the latter, the memmap allocations cease to be page aligned.
This isn't needed though.
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: <stable@vger.kernel.org> # v4.9
Cc: Dave Young <dyoung@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Mika Penttilä <mika.penttila@nextfour.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Fixes: 4bc9f92e ("x86/efi-bgrt: Use efi_mem_reserve() to avoid copying image data")
Link: http://lkml.kernel.org/r/20170105125130.2815-1-nicstange@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>