x86/mm: Set MODULES_END to 0xffffffffff000000

Since f06bdd4001 ("x86/mm: Adapt MODULES_END based on fixmap section size")
kasan_mem_to_shadow(MODULES_END) could be not aligned to a page boundary.

So passing page unaligned address to kasan_populate_zero_shadow() have two
possible effects:

1) It may leave one page hole in supposed to be populated area. After commit
  21506525fb ("x86/kasan/64: Teach KASAN about the cpu_entry_area") that
  hole happens to be in the shadow covering fixmap area and leads to crash:

 BUG: unable to handle kernel paging request at fffffbffffe8ee04
 RIP: 0010:check_memory_region+0x5c/0x190

 Call Trace:
  <NMI>
  memcpy+0x1f/0x50
  ghes_copy_tofrom_phys+0xab/0x180
  ghes_read_estatus+0xfb/0x280
  ghes_notify_nmi+0x2b2/0x410
  nmi_handle+0x115/0x2c0
  default_do_nmi+0x57/0x110
  do_nmi+0xf8/0x150
  end_repeat_nmi+0x1a/0x1e

Note, the crash likely disappeared after commit 92a0f81d89, which
changed kasan_populate_zero_shadow() call the way it was before
commit 21506525fb.

2) Attempt to load module near MODULES_END will fail, because
   __vmalloc_node_range() called from kasan_module_alloc() will hit the
   WARN_ON(!pte_none(*pte)) in the vmap_pte_range() and bail out with error.

To fix this we need to make kasan_mem_to_shadow(MODULES_END) page aligned
which means that MODULES_END should be 8*PAGE_SIZE aligned.

The whole point of commit f06bdd4001 was to move MODULES_END down if
NR_CPUS is big, so the cpu_entry_area takes a lot of space.
But since 92a0f81d89 ("x86/cpu_entry_area: Move it out of the fixmap")
the cpu_entry_area is no longer in fixmap, so we could just set
MODULES_END to a fixed 8*PAGE_SIZE aligned address.

Fixes: f06bdd4001 ("x86/mm: Adapt MODULES_END based on fixmap section size")
Reported-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Garnier <thgarnie@google.com>
Link: https://lkml.kernel.org/r/20171228160620.23818-1-aryabinin@virtuozzo.com

Documentation/x86/x86_64/mm.txt

@@ -43,7 +43,7 @@ ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks
 ffffffef00000000 - fffffffeffffffff (=64 GB) EFI region mapping space
 ... unused hole ...
 ffffffff80000000 - ffffffff9fffffff (=512 MB)  kernel text mapping, from phys 0
-ffffffffa0000000 - [fixmap start]   (~1526 MB) module mapping space
+ffffffffa0000000 - fffffffffeffffff (1520 MB) module mapping space
 [fixmap start]   - ffffffffff5fffff kernel-internal fixmap range
 ffffffffff600000 - ffffffffff600fff (=4 kB) legacy vsyscall ABI
 ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole
@@ -67,9 +67,6 @@ memory window (this size is arbitrary, it can be raised later if needed).
 The mappings are not part of any other kernel PGD and are only available
 during EFI runtime calls.
 
-The module mapping space size changes based on the CONFIG requirements for the
-following fixmap section.
-
 Note that if CONFIG_RANDOMIZE_MEMORY is enabled, the direct mapping of all
 physical memory, vmalloc/ioremap space and virtual memory map are randomized.
 Their order is preserved but their base will be offset early at boot time.

arch/x86/include/asm/pgtable_64_types.h

@@ -104,7 +104,7 @@ typedef struct { pteval_t pte; } pte_t;
 
 #define MODULES_VADDR		(__START_KERNEL_map + KERNEL_IMAGE_SIZE)
 /* The module sections ends with the start of the fixmap */
-#define MODULES_END		__fix_to_virt(__end_of_fixed_addresses + 1)
+#define MODULES_END		_AC(0xffffffffff000000, UL)
 #define MODULES_LEN		(MODULES_END - MODULES_VADDR)
 
 #define ESPFIX_PGD_ENTRY	_AC(-2, UL)