mm/khugepaged: fix ->anon_vma race

In the Linux kernel, the following vulnerability has been resolved:

mm/khugepaged: fix ->anon_vma race

If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires

it to be locked.

Page table traversal is allowed under any one of the mmap lock, the

anon_vma lock (if the VMA is associated with an anon_vma), and the

mapping lock (if the VMA is associated with a mapping); and so to be

able to remove page tables, we must hold all three of them.

retract_page_tables() bails out if an ->anon_vma is attached, but does

this check before holding the mmap lock (as the comment above the check

explains).

If we racily merged an existing ->anon_vma (shared with a child

process) from a neighboring VMA, subsequent rmap traversals on pages

belonging to the child will be able to see the page tables that we are

concurrently removing while assuming that nothing else can access them.

Repeat the ->anon_vma check once we hold the mmap lock to ensure that

there really is no concurrent page table access.

Hitting this bug causes a lockdep warning in collapse_and_free_pmd(),

in the line "lockdep_assert_held_write(&vma->anon_vma->root->rwsem)".

It can also lead to use-after-free access.