Without this patch any kernel code that reads kernel memory in
non present kernel pte/pmds (as set by pageattr.c) will crash.

With this kernel code:

static struct page *crash_page;
static unsigned long *crash_address;
[..]
	crash_page = alloc_pages(GFP_KERNEL, 9);
	crash_address = page_address(crash_page);
	if (set_memory_np((unsigned long)crash_address, 1))
		printk("set_memory_np failure\n");
[..]

The kernel will crash if inside the "crash tool" one would try
to read the memory at the not present address.

crash> p crash_address
crash_address = $8 = (long unsigned int *) 0xffff88023c000000
crash> rd 0xffff88023c000000
[ *lockup* ]

The lockup happens because _PAGE_GLOBAL and _PAGE_PROTNONE
shares the same bit, and pageattr leaves _PAGE_GLOBAL set on a
kernel pte which is then mistaken as _PAGE_PROTNONE (so
pte_present returns true by mistake and the kernel fault then
gets confused and loops).

With THP the same can happen after we taught pmd_present to
check _PAGE_PROTNONE and _PAGE_PSE in commit
027ef6c8 ("mm: thp: fix pmd_present for
split_huge_page and PROT_NONE with THP").  THP has the same
problem with _PAGE_GLOBAL as the 4k pages, but it also has a
problem with _PAGE_PSE, which must be cleared too.

After the patch is applied copy_user correctly returns -EFAULT
and doesn't lockup anymore.

crash> p crash_address
crash_address = $9 = (long unsigned int *) 0xffff88023c000000
crash> rd 0xffff88023c000000
rd: read error: kernel virtual address: ffff88023c000000  type:
"64-bit KVADDR"
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>