mm/hugetlb: don't map folios writable without VM_WRITE when copying during fork()

mm/hugetlb: don't map folios writable without VM_WRITE when copying during fork()

If we have to trigger a hugetlb folio copy during fork() because the anon
folio might be pinned, we currently unconditionally create a writable PTE.

However, the VMA might not have write permissions (VM_WRITE) at that
point.

Fix it by checking the VMA for VM_WRITE.  Make the code less error prone
by moving checking for VM_WRITE into make_huge_pte(), and letting callers
only specify whether we should try making it writable.

A simple reproducer that longterm-pins the folios using liburing to then
mprotect(PROT_READ) the folios befor fork() [1] results in:

Before:
[FAIL] access should not have worked

After:
[PASS] access did not work as expected

[1] https://gitlab.com/davidhildenbrand/scratchspace/-/raw/main/reproducers/hugetlb-mkwrite-fork.c

This is rather a corner case, so stable might not be warranted.

Link: https://lkml.kernel.org/r/20241204153100.1967364-1-david@redhat.com
Fixes: 4eae4efa2c29 ("hugetlb: do early cow when page pinned on src mm")
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Guillaume Morin <guillaume@morinfr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>