userfaultfd: prevent unregistering VMAs through a different userfaultfd

Currently, a VMA registered with a uffd can be unregistered through a
different uffd associated with the same mm_struct.

The existing behavior is slightly broken and may incorrectly reject
unregistering some VMAs due to the following check:

if (!vma_can_userfault(cur, cur->vm_flags, wp_async))
goto out_unlock;

where wp_async is derived from ctx, not from cur. For example, a
file-backed VMA registered with wp_async enabled and UFFD_WP mode cannot
be unregistered through a uffd that does not have wp_async enabled.

Rather than fix this and maintain this odd behavior, make unregistration
stricter by requiring VMAs to be unregistered through the same uffd they
were registered with. Additionally, reorder the BUG() checks to avoid the
aforementioned wp_async issue in them. Convert the existing check to
VM_WARN_ON_ONCE() as BUG_ON() is deprecated.

This change slightly modifies the ABI. It should not be backported to
-stable. It is expected that no one depends on this behavior, and no such
cases are known.

While at it, correct the comment for the no userfaultfd case. This seems
to be a copy-paste artifact from the analogous userfaultfd_register()
check.

Link: https://lkml.kernel.org/r/20250619-uffd-fixes-v3-2-a7274d3bd5e4@columbia.edu
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>