Re: [PATCH v2 hmm 01/11] mm/hmm: fix use after free with struct hmm in the mmu notifiers

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On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

mmu_notifier_unregister_no_release() is not a fence and the mmu_notifier
system will continue to reference hmm->mn until the srcu grace period
expires.

Resulting in use after free races like this:

          CPU0                                     CPU1
                                                __mmu_notifier_invalidate_range_start()
                                                  srcu_read_lock
                                                  hlist_for_each ()
                                                    // mn == hmm->mn
hmm_mirror_unregister()
   hmm_put()
     hmm_free()
       mmu_notifier_unregister_no_release()
          hlist_del_init_rcu(hmm-mn->list)
			                           mn->ops->invalidate_range_start(mn, range);
					             mm_get_hmm()
       mm->hmm = NULL;
       kfree(hmm)
                                                      mutex_lock(&hmm->lock);

Use SRCU to kfree the hmm memory so that the notifiers can rely on hmm
existing. Get the now-safe hmm struct through container_of and directly
check kref_get_unless_zero to lock it against free.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

You can add
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

---
v2:
- Spell 'free' properly (Jerome/Ralph)
---
  include/linux/hmm.h |  1 +
  mm/hmm.c            | 25 +++++++++++++++++++------
  2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 092f0234bfe917..688c5ca7068795 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -102,6 +102,7 @@ struct hmm {
  	struct mmu_notifier	mmu_notifier;
  	struct rw_semaphore	mirrors_sem;
  	wait_queue_head_t	wq;
+	struct rcu_head		rcu;
  	long			notifiers;
  	bool			dead;
  };
diff --git a/mm/hmm.c b/mm/hmm.c
index 8e7403f081f44a..547002f56a163d 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -113,6 +113,11 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
  	return NULL;
  }
+static void hmm_free_rcu(struct rcu_head *rcu)
+{
+	kfree(container_of(rcu, struct hmm, rcu));
+}
+
  static void hmm_free(struct kref *kref)
  {
  	struct hmm *hmm = container_of(kref, struct hmm, kref);
@@ -125,7 +130,7 @@ static void hmm_free(struct kref *kref)
  		mm->hmm = NULL;
  	spin_unlock(&mm->page_table_lock);
- kfree(hmm);
+	mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
  }
static inline void hmm_put(struct hmm *hmm)
@@ -153,10 +158,14 @@ void hmm_mm_destroy(struct mm_struct *mm)
static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
  {
-	struct hmm *hmm = mm_get_hmm(mm);
+	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
  	struct hmm_mirror *mirror;
  	struct hmm_range *range;
+ /* hmm is in progress to free */
+	if (!kref_get_unless_zero(&hmm->kref))
+		return;
+
  	/* Report this HMM as dying. */
  	hmm->dead = true;
@@ -194,13 +203,15 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
  static int hmm_invalidate_range_start(struct mmu_notifier *mn,
  			const struct mmu_notifier_range *nrange)
  {
-	struct hmm *hmm = mm_get_hmm(nrange->mm);
+	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
  	struct hmm_mirror *mirror;
  	struct hmm_update update;
  	struct hmm_range *range;
  	int ret = 0;
- VM_BUG_ON(!hmm);
+	/* hmm is in progress to free */
+	if (!kref_get_unless_zero(&hmm->kref))
+		return 0;
update.start = nrange->start;
  	update.end = nrange->end;
@@ -245,9 +256,11 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
  static void hmm_invalidate_range_end(struct mmu_notifier *mn,
  			const struct mmu_notifier_range *nrange)
  {
-	struct hmm *hmm = mm_get_hmm(nrange->mm);
+	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
- VM_BUG_ON(!hmm);
+	/* hmm is in progress to free */
+	if (!kref_get_unless_zero(&hmm->kref))
+		return;
mutex_lock(&hmm->lock);
  	hmm->notifiers--;

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