mm/uffd: fix vma check on userfault for wp

We used to have a report that pte-marker code can be reached even when
uffd-wp is not compiled in for file memories, here:

https://lore.kernel.org/all/YzeR+R6b4bwBlBHh@x1n/T/#u

I just got time to revisit this and found that the root cause is we simply
messed up with the vma check, so that for !PTE_MARKER_UFFD_WP system, we
will allow UFFDIO_REGISTER of MINOR & WP upon shmem as the check was
wrong:

if (vm_flags & VM_UFFD_MINOR)
return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);

Where we'll allow anything to pass on shmem as long as minor mode is
requested.

Axel did it right when introducing minor mode but I messed it up in
b1f9e876862d when moving code around. Fix it.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Signed-off-by: Peter Xu <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/userfaultfd_k.h

···
 static inline bool vma_can_userfault(struct vm_area_struct *vma,
 				     unsigned long vm_flags)
 {
-	if (vm_flags & VM_UFFD_MINOR)
-		return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);
-
+	if ((vm_flags & VM_UFFD_MINOR) &&
+	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
+		return false;
 #ifndef CONFIG_PTE_MARKER_UFFD_WP
 	/*
 	 * If user requested uffd-wp but not enabled pte markers for