CVE-2017-1000405

The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of
pmd_mkdirty() in the touch_pmd() function inside the THP implementation.
touch_pmd() can be reached by get_user_pages(). In such case, the pmd will
become dirty. This scenario breaks the new can_follow_write_pmd()'s logic -
pmd can become dirty without going through a COW cycle. This bug is not as
severe as the original “Dirty cow” because an ext4 file (or any other
regular file) cannot be mapped using THP. Nevertheless, it does allow us to
overwrite read-only huge pages. For example, the zero huge page and sealed
shmem files can be overwritten (since their mapping can be populated using
THP). Note that after the first write page-fault to the zero page, it will
be replaced with a new fresh (and zeroed) thp.

Notes