x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF

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

x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF

x86 implements page attribute modification using its Change Page
Attributes (CPA) mechanism.

This tracks properties of ranges such as cache mode through x86 page
attributes, and as part of that logic manipulates kernel page tables.

Since commit:

41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")

ranges of kernel page table entries can be collapsed into
huge page table entries as part of this logic.

As part of this collapse, it frees the page tables which the collapsed
entries previously pointed to, and it does so without any relevant locks
being held to preclude concurrent kernel page table walkers.

The only way this code can be reached is if CPA_COLLAPSE is specified, and
this is only set in set_memory_rox() via:

set_memory_rox()
-> change_page_attr_set_clr()
-> cpa_flush()
-> cpa_collapse_large_pages()

Notable users of this are execmem and BPF when manipulating executable
mappings.

However, this is problematic for ptdump as it walks ranges it does not own
and thus runs the risk of a use-after-free on page tables freed underneath
it.

In addition, concurrent CPA collapse operations are possible which can also
cause races.

Resolve the issue by acquiring the mmap write lock on init_mm across the
whole operation.

It is safe to acquire a sleeping lock as all the callers invoke
set_memory_rox() from process context and in any case,
change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a
mutex, disallowing atomic context here.