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

Jason Gunthorpe
2019-05-22T20:12:51Z

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