mm: prevent droppable mappings from being locked

Droppable mappings must not be lockable.  There is a check for VMAs with
VM_DROPPABLE set in mlock_fixup() along with checks for other types of
unlockable VMAs which ensures this when calling mlock()/mlock2().

For mlockall(MCL_FUTURE), the check for unlockable VMAs is different.  In
apply_mlockall_flags(), if the flags parameter has MCL_FUTURE set, the
current task's mm's default VMA flag field mm->def_flags has VM_LOCKED
applied to it.  VM_LOCKONFAULT is also applied if MCL_ONFAULT is also set.
When these flags are set as default in this manner they are cleared in
__mmap_complete() for new mappings that do not support mlock.  A check for
VM_DROPPABLE in __mmap_complete() is missing resulting in droppable
mappings created with VM_LOCKED set.  To fix this and reduce that chance
of similar bugs in the future, introduce and use vma_supports_mlock().

Link: https://lkml.kernel.org/r/20260310155821.17869-1-anthony.yznaga@oracle.com
Fixes: 9651fcedf7 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Suggested-by: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Tested-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jason A. Donenfeld <jason@zx2c4.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/vma/include/stubs.h

@@ -426,3 +426,8 @@ static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
 }
 
 static inline void hugetlb_split(struct vm_area_struct *, unsigned long) {}
+
+static inline bool vma_supports_mlock(const struct vm_area_struct *vma)
+{
+	return false;
+}