mm/hmm: Poison hmm_range during unregister

Trying to misuse a range outside its lifetime is a kernel bug. Use poison
bytes to help detect this condition. Double unregister will reliably crash.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Tested-by: Philip Yang <Philip.Yang@amd.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/mm/hmm.c b/mm/hmm.c
index 2ef14b2b5505..c30aa9403dbe 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -925,19 +925,21 @@ void hmm_range_unregister(struct hmm_range *range)
 {
         struct hmm *hmm = range->hmm;
 
-        /* Sanity check this really should not happen. */
-        if (hmm == NULL || range->end <= range->start)
-                return;
-
         mutex_lock(&hmm->lock);
         list_del_init(&range->list);
         mutex_unlock(&hmm->lock);
 
         /* Drop reference taken by hmm_range_register() */
-        range->valid = false;
         mmput(hmm->mm);
         hmm_put(hmm);
-        range->hmm = NULL;
+
+        /*
+         * The range is now invalid and the ref on the hmm is dropped, so
+         * poison the pointer.  Leave other fields in place, for the caller's
+         * use.
+         */
+        range->valid = false;
+        memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
 }
 EXPORT_SYMBOL(hmm_range_unregister);