Re: [PATCH v4 0/1] Use HMM for ODP v4

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, May 22, 2019 at 01:48:52PM -0400, Jerome Glisse wrote:

>  static void put_per_mm(struct ib_umem_odp *umem_odp)
>  {
>  	struct ib_ucontext_per_mm *per_mm = umem_odp->per_mm;
> @@ -325,9 +283,10 @@ static void put_per_mm(struct ib_umem_odp *umem_odp)
>  	up_write(&per_mm->umem_rwsem);
>  
>  	WARN_ON(!RB_EMPTY_ROOT(&per_mm->umem_tree.rb_root));
> -	mmu_notifier_unregister_no_release(&per_mm->mn, per_mm->mm);
> +	hmm_mirror_unregister(&per_mm->mirror);
>  	put_pid(per_mm->tgid);
> -	mmu_notifier_call_srcu(&per_mm->rcu, free_per_mm);
> +
> +	kfree(per_mm);

Notice that mmu_notifier only uses SRCU to fence in-progress ops
callbacks, so I think hmm internally has the bug that this ODP
approach prevents.

hmm should follow the same pattern ODP has and 'kfree_srcu' the hmm
struct, use container_of in the mmu_notifier callbacks, and use the
otherwise vestigal kref_get_unless_zero() to bail:

From 0cb536dc0150ba964a1d655151d7b7a84d0f915a Mon Sep 17 00:00:00 2001
From: Jason Gunthorpe <jgg@mellanox.com>
Date: Wed, 22 May 2019 16:52:52 -0300
Subject: [PATCH] hmm: Fix use after free with struct hmm in the mmu notifiers

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.

         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@mellanox.com>
---
 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 51ec27a8466816..8b91c90d3b88cb 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 816c2356f2449f..824e7e160d8167 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_fee_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_fee_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;
@@ -248,9 +259,11 @@ 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--;
-- 
2.21.0