page structs are not guaranteed to be contiguous for gigantic pages.  The
routine copy_huge_page_from_user can encounter gigantic pages, yet it
assumes page structs are contiguous when copying pages from user space.

Since page structs for the target gigantic page are not contiguous, the
data copied from user space could overwrite other pages not associated
with the gigantic page and cause data corruption.

Non-contiguous page structs are generally not an issue.  However, they can
exist with a specific kernel configuration and hotplug operations.  For
example: Configure the kernel with CONFIG_SPARSEMEM and
!CONFIG_SPARSEMEM_VMEMMAP.  Then, hotplug add memory for the area where
the gigantic page will be allocated.

Link: https://lkml.kernel.org/r/20210217184926.33567-2-mike.kravetz@oracle.com
Fixes: 8fb5debc ("userfaultfd: hugetlbfs: add hugetlb_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>