arm64: Fix THP protection change logic

arm64: Fix THP protection change logic

commit 1a541b4e3cd6f5795022514114854b3e1345f24e upstream.

6910fa1 ("arm64: enable PTE type bit in the mask for pte_modify") fixes
a problem whereby a large block of PROT_NONE mapped memory is
incorrectly mapped as block descriptors when mprotect is called.

Unfortunately, a subtle bug was introduced by this fix to the THP logic.

If one mmaps a large block of memory, then faults it such that it is
collapsed into THPs; resulting calls to mprotect on this area of memory
will lead to incorrect table descriptors being written instead of block
descriptors. This is because pmd_modify calls pte_modify which is now
allowed to modify the type of the page table entry.

This patch reverts commit 6910fa16dbe142f6a0fd0fd7c249f9883ff7fc8a, and
fixes the problem it was trying to address by adjusting PAGE_NONE to
represent a table entry. Thus no change in pte type is required when
moving from PROT_NONE to a different protection.

Fixes: 6910fa16dbe1 ("arm64: enable PTE type bit in the mask for pte_modify")
Cc: <stable@vger.kernel.org> # 4.0+
Cc: Feng Kan <fkan@apm.com>
Reported-by: Ganapatrao Kulkarni <Ganapatrao.Kulkarni@caviumnetworks.com>
Tested-by: Ganapatrao Kulkarni <gkulkarni@caviumnetworks.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[SteveC: backported 1a541b4e3cd6f5795022514114854b3e1345f24e to 4.1 and
 4.2 stable. Just one minor fix to second part to allow patch to apply
cleanly, no logic changed.]
Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>