[PATCH v2 hmm 11/11] mm/hmm: Remove confusing comment and logic from hmm_release

[Jason Gunthorpe <jgg@xxxxxxxx>]

From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

hmm_release() is called exactly once per hmm. ops->release() cannot
accidentally trigger any action that would recurse back onto
hmm->mirrors_sem.

This fixes a use after-free race of the form:

       CPU0                                   CPU1
                                           hmm_release()
                                             up_write(&hmm->mirrors_sem);
 hmm_mirror_unregister(mirror)
  down_write(&hmm->mirrors_sem);
  up_write(&hmm->mirrors_sem);
  kfree(mirror)
                                             mirror->ops->release(mirror)

The only user we have today for ops->release is an empty function, so this
is unambiguously safe.

As a consequence of plugging this race drivers are not allowed to
register/unregister mirrors from within a release op.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
---
 mm/hmm.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 709d138dd49027..3a45dd3d778248 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -136,26 +136,16 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
 	WARN_ON(!list_empty(&hmm->ranges));
 	mutex_unlock(&hmm->lock);
 
-	down_write(&hmm->mirrors_sem);
-	mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
-					  list);
-	while (mirror) {
-		list_del_init(&mirror->list);
-		if (mirror->ops->release) {
-			/*
-			 * Drop mirrors_sem so the release callback can wait
-			 * on any pending work that might itself trigger a
-			 * mmu_notifier callback and thus would deadlock with
-			 * us.
-			 */
-			up_write(&hmm->mirrors_sem);
+	down_read(&hmm->mirrors_sem);
+	list_for_each_entry(mirror, &hmm->mirrors, list) {
+		/*
+		 * Note: The driver is not allowed to trigger
+		 * hmm_mirror_unregister() from this thread.
+		 */
+		if (mirror->ops->release)
 			mirror->ops->release(mirror);
-			down_write(&hmm->mirrors_sem);
-		}
-		mirror = list_first_entry_or_null(&hmm->mirrors,
-						  struct hmm_mirror, list);
 	}
-	up_write(&hmm->mirrors_sem);
+	up_read(&hmm->mirrors_sem);
 
 	hmm_put(hmm);
 }
@@ -287,7 +277,7 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
 	struct hmm *hmm = mirror->hmm;
 
 	down_write(&hmm->mirrors_sem);
-	list_del_init(&mirror->list);
+	list_del(&mirror->list);
 	up_write(&hmm->mirrors_sem);
 	hmm_put(hmm);
 	memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm));
-- 
2.21.0