[PATCH 06/22] mm/hmm: fix use after free with struct hmm in the mmu notifiers

Christoph Hellwig
2019-07-01T06:22:05Z

From: Jason Gunthorpe <jgg@mellanox.com>

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@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Philip Yang <Philip.Yang@amd.com>
---
include/linux/hmm.h | 1 +
mm/hmm.c | 23 +++++++++++++++++------
2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 7007123842ba..cb01cf1fa3c0 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -93,6 +93,7 @@ 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 826816ab2377..f6956d78e3cb 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -104,6 +104,11 @@ 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);
@@ -116,7 +121,7 @@ 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)
@@ -144,10 +149,14 @@ 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;

+ /* Bail out if hmm is in the process of being freed */
+ if (!kref_get_unless_zero(&hmm->kref))
+ return;
+
/* Report this HMM as dying. */
hmm->dead = true;

@@ -185,13 +194,14 @@ 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);
+ if (!kref_get_unless_zero(&hmm->kref))
+ return 0;

update.start = nrange->start;
update.end = nrange->end;
@@ -236,9 +246,10 @@ 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);
+ if (!kref_get_unless_zero(&hmm->kref))
+ return;

mutex_lock(&hmm->lock);
hmm->notifiers--;
--
2.20.1